I'm familiar with pretty much everything you're running there.
Love the B20s, though my pair was fully P10 modded for me by Dave. They were really good, stock, in OB. They were completely awesome in OB once I cut the dust caps and installed a pair of P10 phase plugs. After the full mod, though, they sounded really odd in OB, I think mainly because the front wave had been so modified and the back not at all that it just didn't match up anymore (of course, they sound ridiculously good in sealed cabs doing HT and music duty in my living room). The lessons I learned were that the B20 sounds weird at certain listening angles if you've cut the dust caps but haven't installed anything (phase plugs I know work great, but you might also try the socket method for something quick, a bit cheaper and easy to source locally, or maybe you can fashion your own plugs somehow; whatever you have to do, its a totally worthwhile upgrade, dollar for dollar), and that wildly modding the front face without doing the same on the rear will likely cut your B20's OB career short. I never had problems with the cupped hand sound, though it would seem with the folk here who report it, the bit of foam or whatnot loosely tucked around behind the whizzer cone pretty reliably helps out.
The APA150 is great for the purpose of being a stereo sub amp. It also gives you lots of flexibility for future experimentation and even a back up stereo amp for any purpose that comes along. I had initially been driving a pair of Dayton RS225-8s with mine (which sounded completely awesome, btw, paired with the even more incredible RS100-8 fullrangers I replaced my B20s with once they got moved to the living room), but they didn't quite have the oomph on OB that I was looking for and then one of them developed a hideous rattle.
At that point, I picked up 4 of the 10" square frame woofers that PE had started carrying at the time (this is like 10 months ago now), partially to play around with a possible cheap OB surround system for my parents' new appartment, but we decided to go another direction so they got added to my RS100s in the office. 1 per side was fine for most music, especially at moderat "work" levels, but were regularly ground out on any DVD content, some more bass agressive music, or even more tame tracks turned up too far. Getting two on a side was going to be problematic height wise, until recently where the desk situation changed to where I've got 1 + RS100 up top around ear level and then a second down below just inches off the floor.
This setup has been very satisfying, low response, higher volume than I can actually listen to in that space, no mechanical distortion. I watch movies, play games, listen to music and even play my guitars (primarily electric bass, no less) over this system. Its not the absolute best I've had in sound quality terms, but its completely capable and reliable to function without any mechanical issues regardless of how I push it, which of course obviously hadn't been the case using just a single 8" per side, regardless of how much higher quality they were.
What that tells me is that with your Goldwoods, you should be able to get similar sound quality to what I have, but you might find yourself hitting some mechanical limits if you're looking for too deep of a response and/or too high output volume. I haven't heard those particular Goldwoods, but just going by the general state of their drivers, specs, etc. I would feel safe in guessing that each one of them is noticeably more robust and capable than one of my square frames, but likely not really close to 2 per side. If you stay within the limits of what the Goldwoods can do, it should sound great.
Other things to note ... You may need to look into a helper tweeter since the B20s don't cover the very top end without some sort of EQ (which may or may not be something you're interested in). Gozilla loves the piezeos (which I haven't tried yet myself) so if you'd like to try one of those, just look up his guidance (you'll need an inductor and a capacitor, both cheap). I've coupled my B20s with very small, inexpensive Datyon domes and with much more expensive B&G Neo3PDR planar drivers (which is what's in the the sealed cases with the B20s in the living room right now), using caps between 2 and 2.5 uF and they always gave immediate gratification and no desire to tweek or upgrade. Also, using the receiver to amp the B20s will probably always give you some hassle or another, the most primary of which is that the sound quality won't ever really let the B20s shine (trust me, I know, having spent like 2 years running them with my old Pio 411, and even my newer and much better sounding Panny X57 with digital amps pulling plows in the living room, there's obviously room for improvement). I'm partial to digital amps, and I have run the B20s with the original Sonic Impact T-amp and then with the second gen one. I could easily recommend giving the Dayton DTA-1 a try since its cheap and should be reasonably similar to the original Sonic Impact its modeled after. Personally I am very interested in trying the newer, higher end and higher powered DTA-100a they have now or maybe something put together from one of their Class T kits. Then if you look around, you'll see lots of commentary from folks with a very nice assortment of tube amps, class A solid state amps, stuff designed by Nelson Pass, and on and on, that could be researched and tried.
Or, and I like this option more and more lately, you can ignore everything I say, and just enjoy the setup you have, because at the end of the day, I know for a fact that it sounds pretty good
Kensai
