The review from Electric House puts the frequency response as 52Hz to 23kHz, which is more in line with a ported 6.5" mid-woofer. For music, you would ideally get that down to 20-25Hz. With your budget, that's not going to happen. 30-35Hz is a more reasonable objective. While the difference between 30Hz and 53Hz may not sound like much, chances are your Episodes do not have the authority (headroom) within their lower range to properly handle the
impact of a kick-drum.
You are really looking for a good woofer with out so much of the "sub" part. However, marketing being what it is, it will be called a sub-woofer.
My budget is what it is though. I may have to settle for something less than ideal that would still get the job done. There's a Polk I'm considering (PSW-10).
Please, please don't do this! If you do, you will think we are all a bunch of idiots for suggesting a sub!
You said:
A stronger sound, but not just "louder," and not sacrificing clarity and detail.
A sub like the PSW-10 is going to crap all over your clarity and detail! But it might be a good learning experience to pick one up from BestBuy if their return policy makes it free! I'm serious, you can learn a lot from a crappy sub!
Given your budget and desire to keep your sound as clean as possible, I think the Dayton Audio SUB-1000 is your best option (I assume you can stretch your budget by $10)
http://www.parts-express.com/dayton-audio-sub-1000-10-100-watt-powered-subwoofer--300-628
I have not heard this sub, but I have heard the SUB-1200 and am very happy with the amount of restraint it exhibits (most $100 ported subs go bonkers when the tuned-port frequency is excited). Since the PE reviews are as high for the SUB-1000 as the SUB-1200, I think it is safe to expect it represents a similar value.
Here is Jman's review of the larger SUB-1200:
http://www.hometheatershack.com/forums/speaker-subwoofer-reviews/71766-dayton-audio-sub-1200-subwoofer-review.html
I can only comment on the SUB-1200 since I have heard it, but the beauty of this design is it is balanced and composed within its capability and doesn't pitch a fit if you try to push it beyond it's limits (it limits its capability to what it can do, instead of trying to "fake" it). If you buy it to vibrate your room for explosions, you will be disappointed because it doesn't dump gobs of indiscriminate energy into your room (the PSW-10 is likely better by this measure), but for music it adds thump without getting in the way of your speakers.
You will not mistake it for a real kick-drum in your room, but the difference will have you doing double-takes and put a smile on your face when you have those "that's different" moments!
Edit - If your budget is really tight, there is also the $96 ($6 to ship - shipping was free on the 1000) SUB-800. It is very well respected by the Parts Express crowd and an 8" sub can add a lot to 6.5" speakers that are trying to cover all of the midrange and bass.