What size subwoofer to get?

I have Spica angelus speakers with a REL strata. The amp, crossover, and subwoofer is all one box. Total cost $500 for the Spicas and $850 for the REL. I don't think I could beat the sound (imaging and bass depth) for $1300.

I think our differing opinions come from comparing one-box subs, crossed below the mains to multi-box subs with external amps and crossovers. A one-box-does-it-all sub like REL or ACI contains the crossover, amp and sub, for anywhere between $800 and $1500, and gives bass of depth I have not heard on any main speakers under $2500 a pair. Creating a sub with separate crossovers, amps and cabinets I have no experience with, but it sounds like your opinions would be more applicable in this case.

How did I conclude you had no first-hand experience ... I read "and decided it was far more cost effective, and would also yield overall" and the wording suggests a decision based on presupposition, rather than a listening comparison.
Also you wrote "As I understand it you should crossover ..." this gave me the impression that you had never actually setup a sub. I guess you post was open to interpretation and I made the wrong interpretation.

Gunbei: I think you are headed in the right direction. Subs in the corner behind the speakers can and often do work quite well. It's not as flat a response, but if you keep them within 1/8 of a wavelength of the corner it minimizes the peak/node effect to some degree (eliminates the re-enforcement of back waves coupling or decoupling to the source wave. As to downfiring or forward firing--it makes no difference at crossover points below 65 Hz or so. Above that you might get a very slight difference. You may want to go to our website www.rivesaudio.com and go to the listening room (under acoustic issues). If you click on speakers there, there is a link that you can download a white paper on speaker placement. It goes through all channels and subwoofer placement. Hopefully, it's helpful.

Sean my experience coincides with your advice. I am using ProAc Response 1SC monitors which probably extend down to 40Hz at the most. But I find I must set my Paradigm X30 to it's lowest setting of 35Hz or the excessive overlap at the crossover point begins to muddy the midrange and upper bass of the ProAcs. That's where an extra control for shaping the slope of the lowpass filter would be helpful in giving more control over the rolloff.

Rives, thanks so much for giving me the free advice. I did check your website and found lot's of helpful information. I've been reading around to become more aware of some of the often used guidelines and from there I intend to do a lot of experimenting. I just wanted to make sure I started on the right foot.

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.