Even as this amp has a balanced input, you can still use it single ended, when you put - negative in to ground.
Then, your pot should work, but as it is a logarithmic (can´t use linear here!) one, you can use it the wrong way.
One side to ground, the other to the source and the middle to the amp input.
That should work. If behaves erratic, change source and ground.
An alternative could be a single ended to balanced buffer after the pot. Will work fine if your source has a strong, low impedance output, like a CD player.
If not, for high impedance sources, use a single ended buffer in front of the pot. As far as I know the 3E-amps even have some voltage output on board.
If you buy these buffers at eBay, might cost you 12$ for small, finished boards.
Only problem, you have to know what you do. If you wire this stuff up wrong, you will get terrible hum.