I found myself in exactly your shoes not so long ago, but with a Denon avr-3300. For improving 2 channel, start by adding a two channel amp to drive your front r and l speakers. In denon's set up menu, you can adjust the levels for each speaker individually. Use the test tone option and a sound level meter (can get one from radioshack for cheap - i think $30-40) That way, you can equalize the volume levels of all of your speakers and there will be no no issues with a more powerful amp powering your mains. I found that adding better amplification to the denon significantly improved the quality of the audio.
From there, you can get a higher end stereo pre-amp with a home theater passthrough. The front speaker pre-outs from the denon would go to the pre-amp, and the pre-amp outputs would go to your new amp. When using two channel, the denon stays off, your two channel source goes directly to the new two channel pre-amp, and you're in business. For home theater, you turn on the denon, use as usual, and have the new pre-amp in pass through mode, letting the denon control everything, as if the front pre-outs from the denon were connected directly to your 2 channel amp. Your denon can handle all of the processing and remaining amplificaion for ht.
After a few purchases of used multichannel amps, I ended up with an Odyssey Stratos for my fronts, and used the denon for powering my center and surrounds. Then I added a Chiro C-500 for the center and surrounds. Still looking for either a new processor to add 7.1 processing or a good deal on a pre-amp with ht pass thru. The denon is solely being used as a pre-pro. Lots of people recommended an adcom gfp-750 preamp for 2 channel. If you are happy with your current ht, buying a 2 channel power amp and a stereo preamp with ht pass thru is the way to go.
Anyway, I'm writing a novel. Start with a good 2 channel amp for the fronts, and move on from there. Good luck.
From there, you can get a higher end stereo pre-amp with a home theater passthrough. The front speaker pre-outs from the denon would go to the pre-amp, and the pre-amp outputs would go to your new amp. When using two channel, the denon stays off, your two channel source goes directly to the new two channel pre-amp, and you're in business. For home theater, you turn on the denon, use as usual, and have the new pre-amp in pass through mode, letting the denon control everything, as if the front pre-outs from the denon were connected directly to your 2 channel amp. Your denon can handle all of the processing and remaining amplificaion for ht.
After a few purchases of used multichannel amps, I ended up with an Odyssey Stratos for my fronts, and used the denon for powering my center and surrounds. Then I added a Chiro C-500 for the center and surrounds. Still looking for either a new processor to add 7.1 processing or a good deal on a pre-amp with ht pass thru. The denon is solely being used as a pre-pro. Lots of people recommended an adcom gfp-750 preamp for 2 channel. If you are happy with your current ht, buying a 2 channel power amp and a stereo preamp with ht pass thru is the way to go.
Anyway, I'm writing a novel. Start with a good 2 channel amp for the fronts, and move on from there. Good luck.