Passive sub with Krell TAS amp?

Need some help, guys. Our theater is wired for several places for subs, but it's with speaker wire, not interconnects. So my husband found a used Krell TAS and bought that to drive the (unbought) sub or subs. (We have another TAS for the main speakers, he planned on using the second TAS for the subs and additional surround speakers.)

I can't seem to find high-quality passive subs. And when I point out that there are good subs with the "high-level" input, he states that they will still use their own internal amplifier and he wants to use the Krell.

So I need some advice. Are there any good passive subs out there? Other options?

Thanks - all of you are really helpful - I don't post often but I read a lot!

Actually, he has not thought of it in that fashion. He wants one "worthy" of being driven by the Krell amplifier. The challenge is that there seem to be almost ZERO truly passive subs out there. He found ElectroVoice (seems to be a car-product and professional theater). I found the second-generation Wilson Watch dog (I am afraid even to ask the price of that).

My challenge is in even finding options. 99% of what I find is powered. So any suggestions are worthwhile and much appreciated.