Joe,
I think, if at all possible, you should take me up on hearing my De Capo I's with the REL sub. If you want, you can run them with your amps/CD player/whatever to approximate your system as closely as possible with the REL. At least then you can be pretty sure that the REL will work for you. I'm always astounded how people can buy stuff based upon what people say on a forum. I'm pretty opinionated here, but if anyone follows what I say here without listening to the piece, then they are doing themselves a disservice, as my listening preferences are pretty different. I guess if you are buying used, then you can use the forums/reviews as a guide and audiogon as the trial stage, BUT you must be willing to sell the piece if it doesn't work out and sometimes people don't want to have to go through all that trouble- or may not want to admit that the piece sounds bad to them, despite all the glowing net/print reviews. If you were never thinking of a REL in the first place, then I guess I wouldn't come listen to it at my place either. However, there are at least two members on this forum that are happy REL users with the De Capo I's, so there is some precedent there.
Whatever sub you end up buying, if at all possible, just go listen to it with your speakers.
Michael