A cunundrum of sorts... any ideas?

@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

2 separate systems

Thanks for all the feedback everyone. I will now do some research on what brands/models I can afford and meet the need.