I think the ß22 I just finished would serve very well as A/B equipment for sources, cables and headphones alike. It has 4 inputs and two headphone outputs.
Source switching is done with a Twisted Pear Darwin, which uses relays instead of the switch contacts to do the switching. They're very close together on the board, so I wouldn't expect any of the 4 inputs to have significantly more transformer-induced hum than any other.
With this amp, the signals would all have the same path from input to output. The internal variables would include the R/L/C tolerances of the relays in the Darwin and the slight differences in wire length between (a) the inputs and amp board and (b) the protection circuit and two headphone jacks.
On top of that, given the ß22's reputation, it's unlikely to have people claiming that it's coloring or distorting the sound. I hope to bring it to some meets, but more to compare headphones than prove audiophile claims right or wrong.