What size subwoofer to get?

Your speakers are very good and extend into the bass regions quite a bit. You do not want to lose any of the benefit of your speakers and there is no reason to try to redesign them by using a traditional, conventional subwoofer. Buy a REL (the size of model depends on size of listening room) and run your speakers full range. Then run the REL off your amp (it draws no current and won't affect anything) and then experiment with the lowest x-over setting you that works. I have ProAc 2.5s and have my REL x-overed at 28HZ. This is a major difference btw RELs and most other systems. I tried, at home, three other subs costing btw 1,200.00 and 2,400.00. The 1,300.00 REL Strata III was far and away the most musical and blended seamlessly with my speakers. You are right to want all the low frequencies you can get, makes a big difference but make sure it's done right. REL is the only make I know that does it right. I've heard Vandersteen's sub does same thing but is far more difficult to set up and adjust.

Since you are looking for music rather than Home theater bombast....try the REL....it comes up under your mains and works off the amp speaker connections.

To add to Rives' comments, front loading and downfiring will have DRASTIC effects on bass output and pitch, even if crossed over at or below 65 Hz. I would suggest listening to the two different designs side by side to see which you like best. My experience is that front firing will typically sound more musical with better pitch definition whereas downloading will provide greater "oomph" with less attack and tonality.

One should also take into account that downloading places more stress on the driver itself ( the suspension can sag pretty drastically over time ) and a system of this nature will require a slightly sturdier amp with greater current capacity / higher damping factor. Sean
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PSB made a very nice sounding passive subwoofer called the Subwoofer 1 in the late 80's early 90's. I just purchased one used for really cheap and it works fantastic with my tower speakers of 86 db sensitivity. (The sub is 88) The speaker level high pass is 6db per octave (fairly shallow) so your mains will be still producing some lower bass and not cause collapse of all the bass to the sub. The sub also has a set low pass crossover in it as well I think. This unit is very musical and ideal for a music system. I found most powered subs being sold today very boomy, high distorting regardless of the convenience of Level control and Crossover controls etc. You have to spend $700 U.S or more to get anything half decent (ie Paradigm PW 2200). My room is similar dimensions to yours and at 100db or less the system sounds excellent. Make sure your main amplifier has adequate power into 4ohm load. This sub is a stereo unit (2 8's) which are high compliance. It can also be configured for mono. The sub seems to be very accurate. Try to find one of these used! I only paid about $70 U.S for mine. The only negative is that it is large 22" X 24" Good luck with your system.

Remember...when buying a sub...you are also buying an amp...the size of the driver is really irrelevant...although some purist find them too "thumpy"...I do think Sunfire subs for 2 channel sound are amazing(Bob Carver claim to fame have always been his amps...and we all know what happened after he left Carver)...a friend has them with Maggie 1.6s...which are notorious for sub mating issues..and they are tight,fast,and very musical...a more laid back type of musical sub can be found by REL...and to echo Sean's comments...a garbage sub will only call attention to itself in a 2 channel set up...and surprisingly...since I am not a huge fan of their speakers...B&W makes some pretty darn good subs too...