What size subwoofer to get?

Socrates .. I disagree with some of your suggestions:
1) If speakers are rated to 35Hz, and you cross below 30Hz (it is usually recommended to leave the main speakers unfiltered, and cross below their cutoff frequency ala REL, ACI) then this will have no effect at all on the stereo imaging ... such low frequencies are not directional.
2) Of the several hundred CDs I have at least 75% benefit from the subwoofer, whether it be added weight in the bass drum kick, the sense of the scale of the venue on live recordings, timpani on orchestral music ... I could go on. The type of music that DON'T benefit are extremely limited. Of course the effect is most pronounced with electronica and organ music, but it is very present in most other types of music.
3) Separating the source of sub-bass from the main speakers gives flexibility in placement to allow good imaging on the mains, and good room loading on the sub. Full range speakers don't give you this option.
4)I have heard good full range mains ... in a large room they work very well. I have heard a good subwoofer ... in most rooms it can work very well. Both options are possibilities. However you seem to rule out the subwoofer approach without having had any first hand experience with one. When you say you decided .. was this based on listening, or just a preconception ?

How do you know that I have no first hand experience? An unusual assumption, no? I have heard numerous setups with seperate subs, I find them inferior in many ways to similar quality and priced full range speakers, though sub setups often end up more epxensive in the end once you factor in cables and a good crossover, which is why it makes little sense to me to use seperate subs in most cases when so many good full range speakers can be had on the used market. If nothing else, why not move up the line of your speaker brand of choice, it will likely cost less, be more aesthetically appealing and easier to setup the gear to boot. I also have to imagine that if 75% of your music is better with a sub then you must have very limited range monitor speakers, which is not the case with the originator of this topic. I'm just offering my opinion, take it for what it's worth, just as others will take your opinion....

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.