Curious, is anyone using Rythmik Subwoofer? I was about to pull the trigger when I decided it might be wise to ask their technical support team and informed I would need a minimum of 50 watts for the speaker level inputs to work properly.

They stated Tube amps do not have enough power to work with their subwoofers.  They explained that a speaker level to line level converter uses a resistance that lowers the input voltage to about 1.5VRms to 2VRms but to get that voltage you have to have a minimum output voltage from you amp. (Wouldn't a preamp address this issue?)

They also stated Tube amps with very low watts produce not enough voltage to get decent input voltage on the subwoofer pre amp then subwoofer output would be reduced to very low levels. I would require a tube amp with more than 35 watts for their sub to work but it would not reach max output.

Honestly, this is all above my comprehension level, but would think that such a limitation would apply to all subwoofers connected to tube amps.

At the moment, looking at the REL T/9x and Kef KC-62 or 92. The REL rep is suggesting the S/510 which I think is too much for my small listening space.

Any input and suggestions welcome.

I'm using 2 Rhythmic F12G subwoofers. Why do you need to use to the speaker-level inputs? Just run the preamp output to the subwoofer plate amp. A potentially better solution is to run the preamp to an active crossover (like the Sublime Acoustic) and split off the LF. This has a few advantages. 1) The subwoofer integration is much simpler as you won't need to match the roll-off of your mains 2) Less stress/excursion on your mains woofers (and hopefully better sound) 3) Slightly lower power requirement for the amp. The disadvantage is another device in the chain (and extra cost).