I bought mine with the Ergo 1 amp. Wasn't bad but obviously not up to the AMT level.
I got Duggeh to send me the wiring diagram of the stock transformer box. It had inductor coils and caps in it.
I then made up a box with a variety of different inductors and capacitors so that I could switch them in/out of circuit.
I have this connected to a 80W class A balanced amp.
I was going to use a signal generator to test the different switch positions and see how they changed the cut/boost of the top and bottom end but never got around to it.
Something to try in the future.