I have a Sansui 9090DB -- about 30+ years old. Still sounds good and the original speakers Sansui SPX 8000 are still rocking.
One of the switches that has about a dozen wafers with contacts and soldered wires has gotten worse over the years.
The unit has separate pre and main amps that are jumpered on the rear of the unit. The jumper is a typical RCA type connection.
So, my thought is to remove the jumpers and plug my MP3 device into the main amp inputs. I have a set of self powered Bose that works like that.
I checked one of the other forums about pre-amp output levels. One guy said that the pre-amp output is variable -- which makes sense. The Bose speakers have a clamp to protect against over driving them. I doubt the main amp does so I'll have to make sure not to over drive them.