What pre-amp are you using for the vinyl?
Are you willing to run the HT through the vinyl pre-amp and amp also? That would be the simplest solution, but it depends on how often the HT system is used versus the 2 channel and how much you are willing to run the Dynaco. You could run front channel pre-outs from the receiver into the vinyl pre-amp. If it does not have home theater bypass, you can put it into another input and just change the volume to a fixed position that balances the levels. Essentially, make your own home theater bypass, with the added step of adjusting the volume to the right level. You can determine that level from the normal volume setup in the Yamaha.
Are you willing to run the HT through the vinyl pre-amp and amp also? That would be the simplest solution, but it depends on how often the HT system is used versus the 2 channel and how much you are willing to run the Dynaco. You could run front channel pre-outs from the receiver into the vinyl pre-amp. If it does not have home theater bypass, you can put it into another input and just change the volume to a fixed position that balances the levels. Essentially, make your own home theater bypass, with the added step of adjusting the volume to the right level. You can determine that level from the normal volume setup in the Yamaha.