What do I need?

If your 2-channel preamp does NOT have a HT bypass feature, I suspect your only option is to directly wire your 2-channel speakers like the rest of the surround speakers from your surround-preamp-processor.  

@kennyc That’s not true.  Any unused line-level input on the stereo pre can work very easily — I did it this way for years. 

@soix I'm doing that with the 2 rear channels for my home theatre. I Bluetooth the rear channels to a Denon integrated amp.  I know where to set the volume on the Denon.  It serves no other purpose other than driving the rear channels, so I just leave it there.  The Denon was sitting unused, so I used it. 

I know Bluetooth isn't ideal, but I bought a good Bluetooth receiver and transmitter with low latency. This worked better than running speaker wires from the front of the room to the back.  No place to hide wires. 

I want HT surround sound for 2001. And for other wonderful composers whose music was used in stereo movies.