Checking the internet, the total cost for proper input expansion boxes would be quite prohibitive since I would require a few such units for my numerous vintage sets.
I have a similar issue with speaker selector boxes .. for another post in the near future.
Then I came across the AV selector below and thought, why not try it?
|View of the output RCA(s) to the amp, pre or processor|
|View of the possible input RCA(s)|
Operation is simplicity itself. Connect the LEFT&RIGHT RCA(s) on the OUTPUT to your PRE or Integrated AMP. Then connect the desired Inputs to INPUT1/2/3 and use the selector accordingly.
An example is the below when deployed on my Quad 33-303 set.
Been using it for a few weeks now and no sign of any undesirable issues or sounds yet ... you would not even suspect the AV selector was in-place unless you peered at the back of the set!