It’s possible. If the 4300 has AL32 processing then it has the same processing power as the 6500. I went from an AL24 X6200 to the AL32 X6500 and I was quite surprised and delighted with the difference. The 6500 certainly has better components onboard along with more headroom. I always change my receivers when the five year warranty runs out, so the 6500 will have to last another four and a half years.
As for connecting a stereo amp it can be very easy depending on the make of the stereo amp. Just a simple RCA connection from the front pre-outs to the direct input on the stereo amp. The front speakers are then just connected to the stereo amp. A simple press of a ‘Direct’ button on my stereo’s remote enagages HT by-pass.
The stereo amp is easily EQ’d into the whole system by Audyssey. The volume of the stereo amp in by-pass is controlled by the Denon. When watching a film or TV both the Denon and the stereo amp is fired up. When you want to listen to music, which should all be connected to the stereo amp, just that amp is in use.
For the sub, you are already familiar with high and low level connection, it would be LFE from the Denon and high level from the stereo amp. I don’t bother with high level from my stereo amp as my speakers go low enough for my tastes.