Which Rel subwoofer

Taters - Personally I would go with a sub that goes the lowest. Especially if you are trying to get what the C1's can't physically do then consider a crossover around 36 - 38 hz. I think you'll find there are not too many recordings that go much lower than what the C1's can do. OTOH if you were thinking of crossing over around 80 hz what you will get is what the C1's can't do BUT adding (probably doubling) to what is already there and may sound real boomy. In that case I would recommend any sub or one that doesn't go very low at all.

I asked which C1's you had because of your response to Pass XA 30.5 with C1 sigs thread. 100w tube amp and wanting more power. Even when I had the originals both the Octave V40se and V70se are more than enough power (even when using EL34 output tubes). What amp do you have? Also for me I don't listen loud and the MKII/Sig is as good or better than the originals when cranked up to sound the best. That being said the MKII/Sig may not actually get as loud (but super close) as the originals before bottoming out.

Xti-16,

I listen very loud so that is why I feel I need more power. If I listened at a low level I would have enough power.My Amp is a Audio Research VT-100MKIII.

Nice amp :)

Before I spent that much coin on the REL-328, which really just adds a sharp spike at 45 Hz, I'd give a hard look and listen to the REL Fathom F110. It's not much more money, and can be had for well under list. The Fathom goes lower, plays louder. integrates easily. and has a flatter, more extended response. The flatter, wider curve makes it easier to integrate.

Johnnyb you mean the JL Fathom F110. Yes the JL subs are special I went from the REL Stentors to a JL Fathom F113 and its faster, goes lower and has less distortion than the REL ever had. Plus no issues, the JL has worked flawlessly where I had reliability issues with the RELs.