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

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.