I beg to differ on it being difficult to integrate a subwoofer into a music system - I've got mine working very nicely thanks (Rel Storm 3 with Harbeth C7/2's). I've also had them working in numerous other setups (including one of the original RELs all those years ago). All done by ear - yes you adjust the settings over time as things settle down and you become used to the sound. The main thing to avoid is setting the crossover and/or the volume on the sub too high. But it's not rocket science as long as you trust your ears. I'm sure having the appropriate measurement kit would give me an even better result, but I suspect the difference wouldn't be night and day.
I'd second the suggestion of looking at the BK 400 - good value at just ove £400 and very flexible. (and yes they used to make RELs). Although eBay can be useful as there are quite a few good secondhand subs on there (where I got my current REL).
One thing to note with RELs by the way is that they are not supposed to be used with a dual mono amp, or more sepcifically anything that doesn't have a common ground. You can get around it, as I have with my Ayre Ax-7e, but get it wrong and you risk frying the amp (as I found out with a pair of Albarry monoblocks a long time ago - oops)