What you are experiencing is certainly NOT due to an impedance mismatch between phono stage and linestage (contrary to Viridian's suggestion). The ratio is about 1:9. That is barely different from 1:10 and anyway at that level there would be no audible attenuation of treble, let alone such an overall signal degradation.
If we are talking about the Clearaudio Performer v2 cartridge, that is an MM cartridge with an output of 3.6mV. That first of all is quite likely to overload an MC phono input, which would cause distortion in and of itself. Your report that the system sounds much better with the phono set to MC and also with the attenuator set for a higher output (less attenuated than previously) does not make any sense at all. When you had the cartridge connected to the MM inputs, was the load resistance also set for 47K ohms? Because if you had it driving the MM inputs but the load was set for a typical MC cartridge (e.g., 100 ohms), that WOULD sound horrible and might account for your original complaint. Meantime, have you consulted the seller?
