I also have some KT88s (Genalex) that I got with another amp. I have used them in my Rachael for a day or so just to make sure that they work as a potential back up. For now, I am sticking with EL34s just because I want to.

Ron Sheldon at cryoset likes the genelex kt88. I believe he said they were the best he sold. No noticeable hum on my system from the listener position.  Only 89db speakers though.

Running a Rachael with VCaps which Btw, I did a side by side with my other Rachael non-vcapped and its a big improvement in openness and soundstage.  I'd like to do the same comparison with jupitors.

The meters are just a hair over 42.

I think the biggest difference is in the bass.  Much tighter and more powerful with the KT88s.  The mids/highs are smoother than the EL34s I have (two sets) .Perhaps a bit rolled off, but it works well with my full range drivers.

I know someone on this forum as well that prefers th KT77s.

I don't know the technical answer but as I mentioned above the bias meters read higher and they sound louder.  

I did a quick testa at time I checked the room with a meter and didn't see a difference.

I need to do a better test with a meter to verify.

I don't know much about the topic either, but it seems to me that narrowing your reference tone to only 100 Hz is not representative of the real frequency distribution typically exposed in normal music listening.

The result would be constricted to the 100 Hz realm only, thus not a reference you could extrapolate to the rest of the distribution spectrum. My $0.02

I found this description of the KT88 from the Tubestore interesting:

"In general, the KT88 tube has a magical mid-range similar to the smaller EL34 tube, but with low and high end extremes that reach much further."

And this from an audiogon post comparing EL34 and KT88:

On sound quality, the consensus seems to be that the '34 has more lushness or richness in the midrange than almost any other mid- to hi-power output tube while being a little soft at the frequency extremes. The '88 (and its very close cousin the 6550) is more balanced across the frequency band, exhibiting more-extended and -controlled bass and better-quality treble than the '34 while lacking the '34's (euphonic?) midrange richness. These differences probably can be swamped by the extremes of different brands of the 2; the above is average or typical.

I'm not sure that is the case with ALL EL34s or all amps they may be plugged into. I've seen individuals online making broad sweeping generalizations like this by using a half a dozen tubes in one machine.

I agree. I think the EL34 in general ought to be more friendly when it comes to room conditions, relative to KT88's, precisely because the lower and higher frequencies are more aggressive when bouncing around against the room walls and such.

From these frequency distribution tails, the lower ones are the most difficult to manage successfully. This hobby is really tough and exciting.