Dear Howard: First than all: congratulations for your North great speakers.
+++++ " Is it possible to buy a high-quality sub for relatively little money? I can't bring myself to spend $3-5k on a pair of speakers only to turn around and spend nearly that much on a sub. " +++++
The task that have a subwoofer is the most difficult area in the home audio frecuency spectrum ( from 80Hz down flat to 16-20Hz ) sound reproduction. So ,you can't expect that that " very hard task " is for a " little money ", at least not for a good quality sound reproduction.
If the task of low/deep bass reproduction were an " easy " one then the subs does not exist because any builder/designer of speakers could incorporate in their full rage speakers and for a low price. If you take a look to the price of full-range speakers ( 20Hz to 25kHz ) you can " see " that everyone had a very very high price. Much of that high price has to be with the quality reproduction of the last two octaves in the music.
Now, yours North speakers deserves something, at least, at the same quality sound reproduction level: not less. Well this is my opinion.
I agree with the people that is in favor of the sealed subs and agree, too, with the people that is in favor of two subwoofers instead only one.
I'm in favor, too, of self-powered subwoofers instead the passive ones. The self-powered subwoofers have many advantages against the passive ones.
Now the issue of down firing against the front ones it is not important in sealed subs.
Now, your main speakers needs a " true stereo " ( a pair of subs )integration subwoofers. The REL ones are not a good option for you: the REL design is for " bass reinforcement " on true full-range speakers or for HT, not for a true stereo configuration with monitors liker yours.
Now, the assumption that the subwoofers objective is to enhance the low bass reproduction is a misunderstood about. The integration on a true stereo fashion of two subwoofers in any system has to be more with obtain not only a better quality low bass reproduction but: better mid bass, better mid range and better high frecuency reproduction.
Please if you have the time read very carefully these links:
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&27&4#27
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&31&4#31
+++++ " 50hz and below...the lower the better. " +++++
+++++ " many are rated even higher than my 150lb. Krell..." +++++
Dave, if you can read those links about.
Btw, of course that many of today subwoofers are rated much higher than your Krell amp, but the task of your Krell is very different that the one of the sub-amp: you can't compare it, the 150lb means nothing in this subject.
Regards and enjoy the music.
Raul.