Receiver +2ch pre on same speakers? Which one?

You can just get rid of the three channel amp. If you have a pre-amp(with processor loop) and a main two channel amp, just power the center and rear surrounds off of the receiver. Run the receivers L&R front pre-outs to the input of your pre-amp processor loop. For music, just use your source (connected to the pre-amp) and the preamp/amp. For HT, engage the processor loop and the receiver will power the center,rears,LFE sub from your digital source. The outputs from the receivers L&R front pre-outs are connected directly to your main amplifier when your processor loop is engaged.

This is how I have my system set up. I have a dedicated two channel system with an HT receiver add-on through the processor loop of my 2-channel system. The receiver powers center, surrounds, LFE sub.

For a cheap adaquate HT receiver, you can use any receiver as long as it has pre-outs for at least the front L&R speakers. I've had Sony STR-DB930, Sony STR-DB940 and Denon AVR-3300. All were good. IMHO, the Sony's had better ergonomics (easier to use) but the Denon sounded slightly better. I have a B&K AVR-202 on the way. FYI, I will be listing the Denon for under $400 in the near future.

Enjoy,

TIC

You could just get a speaker switching box. I know it is not a perfect way to go, but it will get the job done. There are some better made ones out there. Just a thought.

I'm currently going through the same process. Downgrading HT to go back to stereo. I bought a stereo preamp with surround inputs (which you say you already have, right?). Then I just bought an inexpensive Denon receiver ($375). It has preamp outs, I'm going to run the F/R and F/L pre outs on the receiver into the surround L and R inputs on my preamp. This should allow me to use my mains as the front channels in HT. I say should, because the Denon doesn't arrive until tomorrow, 4/1.
You should be able to do the same thing with your processor loop. Just make sure you buy a receiver with preamp outputs.
That high end HT just became a nightmare for me. Stereo is so much easier, and 80% of my listening is 2 channel anyway. It became too expensive and counterproductive to try to go SOTA in 6 channels. I spent more time scrolling through menus and running test tones, than listening to music.
Good luck,
John

PS: The receiver I bought is a Denon AVR-1802.