Sorry to go on and on in this. There are actually a couple of S1200U 3D models up for sale on usaudiomart for $2800. Wow! low price, actually.
In most cases I agree with atmasphere's comment that a separate 2-channel preamp will be better (in most cases) than any HT Processor. However, this Krell S1200 with my power supply mod is pretty freaking amazing, in my opinion. I would actually put it up against many other 2-channel preamps (unless we are looking at Krell Illusion/Phantom with separate power supply level).
The S1200 does not support 4K video over HDMI (only 1080p). If you don't care about this, then I would say S1200 would be the best ht processor choice and also great preamp for 2-channel analog sources.
Atmasphere also commented that 5 channel amps have shared power supply. That is definitely a restricting element if you watch high volume movies. When you are listening to 2-channel music only, it is less of a problem. However, a separate Parasound A21 for left/right would definitely be an improvement. Though, an S1200 preamp/processor would be a better first step.
Oh, and upgrading the main fuse in the A51 or A21 amps to Furutech rhodium will increase the body and make the sound less thin.