You're raising the volume level of the preamp simply because those two amps have different sensitivity specs, which is fine, and which is why the preamp has such a wide range of output (0-99). My C220 is set at 35-45 for most listening, up to 50 on occasion. (I have all inputs trimmed to 0.0dB.) I don't think you've got anything to worry about as far as distortion in the tube stage, and those tubes hit a solid-state buffer before they go to the amp (which is why the C220 can drive amps with unusually low input impedance). Anyway, how's it sound? Does the MC152 win?