Which Rel subwoofer

I'm not familiar with either of those but I have a Rel B3. Great combo but I did replace the stock Rel cable. Better attack and decay. Which C1's do you have? Originals or MKII/Sig? When I upgraded from the original to the Sigs I had to cross over the Rel a little lower to keep the sound 'balanced'.

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.

Nice amp :)