Hey roggae,
I get your frustration. It seems the Torii and source are not an easy bass fit with your speakers.
I believe you can get it though...
I see your speakers are 12 ohm. Is your Torii set up from Decware for 8-16 ohm impedance, or 4-8 ohm? I don't really understand how this works, but it seems the way the Torii mates with speakers, especially for bass, can be more of an issue with some speakers than others. Have you called Steve and asked about the your Zus with the Torii? He may have some tips.
My amp is switchable 4 and 8 ohm output and my speakers are rated at 4 ohms. As I hear it, if the output of the Torii is set for lower 4 ohm impedance (less resistance/more flow)...the sound is bigger and fuller. For 4 ohms, the impedance switch (the one just in front of the speaker cable terminals) is set toward the back of the amp. Set the switch toward the front of the amp, the higher 8 ohm setting, the sound is cleaner, less big...the bigger ohm number for amp settings does not mean bigger sound....
It could be worth double checking the impedance switch and see if it helps one way of the other. I would think toward the front would be your choice to help solve bass issues. You can switch it while playing music, and choose the setting with less bass issues for a baseline.
The pics help. It looks like there are some sort of feet holding the speakers up off the platform? I gather Zu uses an atypical porting system. And I can't really see what is underneath but there looks to be a nice space between your isolation platform and speaker base, and looking back under the speaker base, it looks relatively deep. So it looks good, and easy, for experimenting without directly messing with anything under the speaker.
That gap between the speaker base and your isolator platforms is what I am talking about plugging to see if you can musically reduce low bass. I think it is likely to sound better if you do....the speaker bass loading design looks like it is "venting" low bass out the bottom. If you adjust with regulators tubes, this would help, and is likely necessary as well from the sound of your bass build up problems. But they reduce the push/intensity to the inputs and power tubes pretty much across the spectrum. This shows up more as reduced bass, but will change the mids and highs as well, opening them up and mellowing them a bit...This is relatively subtle done incrementally, and will probably be good for your overall sound.
But trying plugging the bass ports is limited to the problem of bass only, so a more direct and I would hope, a more powerful approach to fixing the things you don't like. Just by reducing bass masking, it would open the mids and highs too, but the intensity of the signal will stay the same. This is why I recommend trying speaker adjustments first, and you can always pull the plugs out! I have done everything, room treatments, power, tubes, cables, and EQ, and love my sound. And judging from how bad your bass buildup is, you will likely want to do both tubes and speaker adjustments if you want to get really close to the beauty, but doing it incrementally is good learning for future refinement.
The thing I tried to remember before I became a died-in-the-wool "adjuster," is that this is your system/room. And no matter how good the designs of your components, they are creating an unbalanced problem with each other, and with your room. Together, they are messing up your musical pleasure. You should be able to get great beauty from your system/room, it just might take more work in different areas. Especially since a drone bass sound implies pretty heavy issues, you probably have to cover more areas.
So this is where I would start: Close up that space between the isolator platform and speaker bottom, trying to calm the level of bass bass coming out the bottom of the speakers to less powerful/more musical levels. It looks like about a 1/2" vertical space. Is that about right? Some "backer rod" foam would work well, enough bigger than your space height so that it will hold well after you gently press it into the gap.
This
http://www.homedepot.com/p/Sika-3-4-in-Closed-Cell-Backer-Rod-108130/202523820 might be about right...or whatever size seems good based on the actual gaps height. All building supplies carry it as far as I know, and it is cheap.
Omen specs say they they are 12" square at the bottom. Bass being soooooo powerful, I would try a fairly dramatic adjustment at first just because here, with my speakers, I had to close them down a lot. I suspect this will be similar there, but maybe not.
Not knowing how the port setup is arranged, it is hard to guess the absolute best way to go. Could be worth a call to Zu and see if they have any ideas, or if there are limitations you should be aware of.
If it is open evenly all around now, my thought would be to plug all sides evenly...leaving vent spaces in the corners, but doing substantial blocking...hopefully enough to reduce low bass nicely.
I might try 8 pieces cut to 10" long or so, one for each side, centered within the 12" width....pressing the foam in with the big side of a chop stick or dowel or something else dull and wide enough, but small enough for the job. I would start just barely pressing the foam into the gap so that the outside of the rod foam is just a little deeper than flush with the outside edge of the speaker base, the inside not directly interfering with anything. With about 1" on each outside edge open, each corner would add up to roughly 1.5" inside spaces for venting bass. Plugging one edge at a time and listening could be instructive. Starting in the backs, then sides, then front would be interesting to listen to as you go.
Too much bass cut? If the bass level is close to good, I would take it really easy. Here, the point at which it gets "just right" is an amazingly narrow window, after which, a 1/2" too much opening gets more muddy. Since the closest room corners load bass more, maybe start by opening the front space 1/2" or so, leaving the back as is? Still too much, maybe try cutting 1/2" or less off the fronts of the side pieces, opening the front corners a bit more....a little at a time...
Alternately, if the original even blocking setup with about 10" pieces is not enough bass cut, it might be intersting to totally plug the backs, filling in with a 2" piece. Then if that is still not enough, maybe slide the side pieces back enough to touch, or nearly touch the back piece...closing the back corner gaps, the front space on the sides now bigger, but the more bassy back smaller....
The thing to me would be to incrementally close the bottom, keeping tuned in to any potential problems as I go....and adjusting if need be. My thought would be to try and get to where bass is reduced to good levels while still remaining musical, and still allowing port venting.
If you were to put me on the spot to guess tubes based on your report, I would lose the OA3 first...the big voltage regulator that effects the push intensity of power tube. I use OB3s in my main room, one step toward more-open/less-bass compared to stock OA3s. But I really like OC3s (a step further into less bass/more open) using them before I got into seriously refined EQ in my main room. I still use OC3s in my Torii III in my work room. And my bass buildup was not as intense as yours sounds, so the OC3 is likely a safe bet and you might even need OD3s, the furthest into open/less-bass you can get with these Regulators.
Old Hytron OC3s are really nice, nuanced and transparent sounding...
These look good to me and wouldn't hurt too bad if they don't suit you:
http://www.ebay.com/itm/Tests-NOS-Date-Matched-Pair-CBS-Hytron-USA-JAN-CHY-0C3-O...And for chilling the intensity/bass at the inputs, I heard first from Lon about Tesla OA2 equivalents....good sounding tubes that will calm the input tube intensity musically...with warmish texture. They are less pushy than your stock OC2s (the little Voltage Regulators in front), and will do a similar relaxing thing as the OC3s will do for the power tubes, but differently because the inputs have a different effect on the overall sound than the power tubes...
I suspect you may want to use both the bigger OC3, and the little OA2 together, but if one happened to be enough, having both to choose from for variations on the same theme could be a nice tuning tool. If you order both, it is not much cost/risk for the pleasure they could potentially provide.
http://www.ebay.com/itm/2-X-11TA31-TUBE-OA2-TUBE-0A2-TUBE-TESLA-BRAND-TUBE-NOS-2... This lot being from a big box, I would ask them for a pair that matches in construction, appearance and tests if possible.
Hope this helps! If these tubes are not enough, there is more, but rectifiers and inputs are more expensive and regulators might put you in a good place!