i left it now as I have no choice to try the Rel and if I do not like it they said they will do a special order on a SVS SB2000 from one of there suppliers .
Firstly, RS aren't an SVS dealer, so can't sell SVS with an official warranty. If they were, they'd have them on their website.
They said they have had no complaints about the Rel T71 and when I talked about the SVS sub they said it was not all about power just because the SVS sub is 500wtts it depends on how define the SVS is and not just about noise and then I said it will go down lower then the Rel they said you probably would not hear that difference we can sell you the Dali k14 £950 which is 500watts and the same inputs as the SVS .
To get a subwoofer to dig deep takes a few things, other than being well designed of course. It needs a good quality driver that is capable of cleanly reproducing the frequencies aimed for. Secondly, it needs enough amplifier power to reach those lower frequencies and keep the driver under control at such high excursions - it doesn't matter if it was an 18" driver, 200w isn't going to allow it to reach anywhere near as deep as it is capable of, and even if it did, there'd be very little control at those lower frequencies. So power does matter. Some manufacturers tend to put in lower powered amplifiers because the average sub buyer isn't interested in how deep it'll reach, most are after a matching wood finish. It doesn't need to shake the house, it just needs to make itself known. That's actually not what you want!
A subwoofer should be so clean sounding that you can't tell where the sub is. It should sound like the sub bass is coming from the speakers in the room. If it's not, it's either not very well set up, or it's producing distortions that are giving away its location. Or possible because it's been placed right be the side of the sofa, where you can't fail to hear/feel where it is. Many times I can't tell if mine is actually on, until I hear a deep note that I know the speakers cannot reach!
Inputs and outputs are irrelevent as long as it has the inputs and outputs you need, which usually boils down to a single RCA input.