Upgrade path question with $2k to spend

@davedword short answer is if you replace your AVR with 2-channel, you can only play back 2 channels (L-R stereo) of a multi-channel recording.  Consider though that most multi-channel audio releases also include 2-channel stereo.  Frankly, most of the re-channeling of old releases isn't all that great (unless we're talking Pink Floyd), and you'll get more enjoyment out of a great 2-channel system with millions of available recordings than mediocre multi-channel with only a limited amount of available source material.

@icegeek , this one is easy. Get a second sub and  proper 2 way crossover. This will get your system stompin for sure. Everything else in the chain is minor in comparison. It is always at the back end of the system that improvements are most noticeable. If you pull 80 to 100 Hz away from your focals you will lower distortion and increase your headroom as much as 10 dB. That is twice as loud. Clipping will be much less common. You ALWAYS need two or more subwoofers. It takes at least two to smooth out room irregularities but the more the merrier. The bass in the room will be more even and powerful. 

Forget the amp for now. You want the bass out of those Arias. The result will be even better. You would need an amp 8 times as powerful to get the same result. We are also not talking about just volume but clean volume. When your system can play at higher intermediate volumes (90 dB) super cleanly the experience is a lot more like being there. 

OP,

You have to be a little careful using watts per channel. A good audio amp with high current could provide a much better drive and sound better with much less wpc. Indirectly the current is indicated by the wpc delivered into various impedance. A great amp with high current will double the watts per channel from 16 to 8 to 4 ohms. So, perhaps 100, 200, and 400Wpc. High end audio stuff will have much higher performance for the specifications, the noice floor will be much lower… which will be satisfying at a much lower value. Surround processors put out a lot of noise and require cranking up to sound good.

To cover all the facets of getting where you want to go I recommend getting a copy of Robert Harley’s book The Complete Guide to High End Audio. You have a sense of where you want to go, and getting there on a small budget is definitely possible. But it will require very carefully chosen components. I highly recommend used ones made for high end audio.

Since this on Audiogon, it should go without saying that you can get more for your money buying used, even at these semi-inflated prices. Lots of options.