Hi!
I am not surprised. The Colibris have a too high output impedance for the built in MC step up. My phonos work best with cartridges of 10 Ohms or less output impedance. For higher impedance cartridges the built in transformer can be rewired or replaced by a better matching one.
Thomas
What's the internal impedance of the Master Signature? This isn't one of the parameters I've seen written on the inside of the box. I'm also curious about SUT matching. Thanks.