I run outputs from my sources (TT thru a phono eq, the 'puter, CD, tapes) into a matrix which goes directly either into the main input of an amp and/or to a receivers' tape in. Sometimes an outboard eq gets 'looped in' via the matrix for input level control and/or eq, depending on the source.
This routine cares less whether both or either are running.. there's a slight drop in input level if both are running, but it's nominal....
Maybe I'm just lucky, or I'm not experiencing the 'feedback' that onhwy61 notes...any thoughts on how this works vs. the OP's state?