Only use part of the receiver, any problem...help

I'd first suggest using the tape loop output from the MD, assuming you have one, rather than the pre-out, which is really meant for situations where you use the receiver as your control preamp and want to use another amplifier. This would bypass the MD's preamp circuitry and volume control, and therefor give you less circuitry in the signal path; also you would not have to use more than one volume control (usually tape outs are before the volume control in the circuitry). As far as what will happen in the long run, since MD gives you a pre-out option, and it's solid state, I would not expect any problems from the amplifier section being on, although you might want to check the owner's manual or call MD to make sure you don't need to do anything special with the amplifier section. Of course, if you call MD, they'll likely suggest either you're nuts to bypass their fine preamp and amp, or else that you sell the receiver and purchase the equivalent version in their standalone tuner line!