A cunundrum of sorts... any ideas?

For $2900 you can get this AVR and center/surround speakers. Add another pair of interconnects and speaker cables for the speakers and you’re all set. Run interconnects from front L/R pre outs of the AVR to an unused input on your stereo preamp, and the AVR powers the center/surround speakers and handles HT processing, but when you listen to 2-channel it’s completely out of the signal path. All you do is change the input on your preamp to switch between HT and stereo. Easy peasy.
https://www.accessories4less.com/make-a-store/item/yamrxa2080bl-rb/yamaha-rx-a2080-9.2-ch-x-140-watts-a/v-receiver/1.html

https://www.aperionaudio.com/products/verus-iii-grand-center-channel-speaker

https://www.aperionaudio.com/products/verus-surround-dipole-bipole-speaker-pair

This is exactly what I do and exactly what HT bypass does. Most people don’t understand HT Bypass.

HT Bypass is a way of sending the preamp signal from your HT system for the front 2 channels to a seperate input on your 2 channel amp. It is like switching to a different input when you want to watch a movie. You switch back for 2 channel sound.

The only thing you need is to make sure your HT system has pre-out for the front 2 channels and most all of them do.

One clarification. I don’t think I’ve seen an HT system with a digital out so you won’t use your DAC. However, the DAC in a good HT reciever is fine for movies. You’ll use your normal DAC for 2 channel sound.

Jerry

I will add that my amp does not have HT bypass. So I added it. HT bypass is an input directly to the amp. I added a set of RCAs and a couple of quality toggle switches so now I have an extra input for my HT. However, lots of preamps and amps have HT bypass. In SS I highly recomment the Kinki Studios EX-M1+ with a very nice HT bypass and excellent sound. clear like a tube amp. Yes, I’m a tube amp guy, we aren’t afraid to mod our amps.

@carlsbad that list is missing mine.

I was surprised when I saw a bypass on the used preamp I got, and in my case I do not need to power the preamp…

• when the preamp is shut off then the bypass is relayed “on”.
• If the preamp is on, then bypass is off as the relay points back to the preamp inputs.

But one either needs to turn on the amp(s) manually, or have a trigger input etc. like on a newer amp.

The only thing you need is to make sure your HT system has pre-out for the front 2 channels and most all of them do

^this^

I think @carlsbad missed the point here.  Clearly the vintage preamp you’d like to use will not have an HT bypass feature, so what you’ll need to do is set a reference volume point on your preamp when you set the 5.1 volume settings so you can just set your preamp to that level when switching to HT.  It’s not hard, but an HT bypass is not necessary to make this work.  I did it this way for years before I got a dedicated listening room and it worked really well.    

@soix  Yes, I thought he was shopping for equipment.  What you suggest will work but isn't as convenient.  I have one tip. Don't ask me how I know.  Frankly I rejected it and insisted on HT bypass.  But sometimes I"m a bit obsessive, I admit it.

Your point on setting a reference volume is very important.   An HT bypass sets the volume at 100%.  Many people try to emulate this by setting the volume at 100%.  DONT DO THIS.  

Eventually you will switch back to music and forget to turn it down.  Choose some volume that won't break anythig in your system or your ears.  Your HT system will have a balance setup you have to go through anyway  (some have microphones and autobalance).  Go through the setup with the 2 channel volume set at the reference volume.   Pick something loud so that the HT preamp isn't maxxed out, but something that you're ok playing music at.

Jerry