Did you say you have two 4 channel amps?
What kind are they ?
Some one else was on here asking about the PDC recently I think
the solution was to combine the left and right front and rear channels
with a Y before feeding them to the MS-8 .
I don't see why you would want to put the stock amp back in under
any condition. If you don't have enough amp channels for every speaker you could always run the rear's off the ms-8 alone.
Somebody needs to do and MS-8 sticky
There are more threads like the one above try search = ms-8