Amp/Preamp Asylum: If the transformer to which you refer is an input transformer, yes, but if not, its still possible:- by Ralph

-Even if the amp has just a single ended input and no input transformer, it is still possible to have it process both phases of the balanced input (although technically it will be differential and not balanced).

If pin 2 of the XLR is driving the same input as the RCA connection, then pin 3 could be used to drive the cathode of the input tube, via a coupling capacitor. This cap would be of the same type and ratings as a cap that might be used as a cathode bypass capacitor for that stage.

This works quite well, even if the cathode is the same point to which the negative feedback is applied (if the amp has negative feedback).

The hardest part of this technique is finding a spot to install the XLR connector and then installing it. The hookup itself is really simple, as all that is needed is the electrolytic cap for the pin 3 connection. If the amp already has a cathode bypass cap installed, then you are all set- simply move its connection from ground to pin 3. If you do that, please note that a jumper pin from pin 3 to ground should be used if you are using the RCA input.

This approach does not change the character of the amplifier at all but it does allow it to process both phases of the input signal.