Subwoofer Challenged - Want to add passives...

OK, I have a high thread-death rate so I'm hoping that I can lure someone in here to post an answer I can sink my teeth into...

SYSTEM: ONKYO TX-SR605
QUESTION: What do I do to add passive subwoofers to my receiver?

Monoprice recommends grabbing a monoblock, of which I don't even know what that is, and powering the passives through the monoblock which is then somehow tied into my Onkyo. How? What kind of block should I get? Obviously since I have an entry level receiver and I'll probably be picking up monoprice speakers, I'm not going to be looking for the most high end solution but I would like something to round out the bass in the room.

The current, powered subwoofer is about 15' away from the central viewing area and I was thinking of adding one or two in-wall subwoofers low to the ground behind the couch. But before I can even decide if it's a good idea, or adequately express _why_ think it sounds like a good idea (just to round out the low end of the mids, really) I'm trying to learn _how_...

And for as common as this question seems to be, most of the answers seem either vague or contrite - such as hooking them up to the bi-amp/etc. as a hack. Even if you just throw links at me for directed research, I promise I'll dig into them...

I'm just looking for the proverbial clue-card to get me started.

I don't know if you mean that acoustically - in that I could make the whole wall rattle and cause the nails/etc. to resonate horribly - or that I don't come across as competent enough to run them in the walls... but I'll go with the former since I have plenty of experience pulling wire I just don't know the first thing about quality sound.

Ultimately, because I can't set up a 7.1 system in my living room in a practical format, I was just hoping to get a little extra "omph" by placing a passive sub in the wall about a foot off the ground directly behind the viewer's couch and, in doing so, create a 5.2 system - where the other, powered sub is the the one that shipped with the ONKYO system mentioned above.

Here, let me ask a slightly different question along the same vein...

Where would you point someone if they were looking to get a crash course / beginners lesson in sound dynamics and how to recognize common mistakes...

The honest answer is - I'm putting monoprice speakers in my wall for my system and 7.1 won't work right because my couch is against the back wall with no space for the sound to travel, therefore as a compromise I've been considering a 5.2 set-up under the premise that using a second sub to bridge the gab between my active off in the corner and my mids all around me, I can have a fuller/richer/deeper? sounding experience than a simple 5.1 system can provide.

This becomes even more true as I have to compromise more and more on my 5.1 speaker location because even the location of my surrounds is a PITA because there are door frames on both the left and right side of the couch, about 8 ft. away in either direction which means I'm either back to a wall mount or mounting them above the doors with toe-angled down a little bit towards me.

I am the tiniest bit concerned about an in-wall sub causing resonance in the wall (nails vibrating/etc. but only because this house is a POS, lol. Still, for the price of a monoprice in-wall sub, it seems worth the at least testing out, especially if it can kick up the experience a little. However, before I did anything spendy, I figured I would at least try to learn about it more as well as run the idea past the masters here.

---
Is that completely fabricated and senseless? I'm still trying to work out the placement of my surround so I haven't even ordered this speaker yet if it's an absolutely terrible idea, but if it is, I'd at least like to _understand_ why...

My current sub is a 10" 230w piece of out of ONKYO SKS-HT750 which means not only is there a lot of room for improvement but also that there probably isn't much resell value in it, lol.

I think, before I upgrade my single sub, I'm going to upgrade my SR-605 to something that supports two sub-outs as well allows me to use my rear-surround channels as front-highs to give me better sound all around. Not to mention HDMI 1.4a on the video side of things.

Like I said, I'm just getting started in the realm of sound so once I have a receiver that's a little more "on par" I think I can start investing in more quality speakers. I've bookmarked that HSU MBM-12 for the next round of purchases, however...