Well since your preamp is a hybrid with no feedback, it probably does have a highish output impedance and the 2100 has a 10x higher input impedance than the 7100 but I think that in terms of the overall affect, the lack of dynamics of the 2100 will be a far greater impact than any possible impedance mismatches you may encounter with the 7100. I bought a 2100 one one time for a ridiculous deal on ebay. It was nice, as was the 2125, but not as satisfying as the 7100 IMO. The little 7100 is really a very nice amp.
Things have changed since the 70s in terms of part quality and so the sound has effectively "cleaned up." This is one fo the first things I notice when comparing an older amp to a much newer one in the same system. It cannot really be overlooked. I will spare all the details but the upshot is that I would still suggest you would be happier with the 7100 (and you are right - it is very slick looking - like my C712). Let us know what you decide! The 2100 is indeed cheap if you want to experiment with it and see what you think - resale is not a problem with McIntoshes. Good luck! Arthur
Things have changed since the 70s in terms of part quality and so the sound has effectively "cleaned up." This is one fo the first things I notice when comparing an older amp to a much newer one in the same system. It cannot really be overlooked. I will spare all the details but the upshot is that I would still suggest you would be happier with the 7100 (and you are right - it is very slick looking - like my C712). Let us know what you decide! The 2100 is indeed cheap if you want to experiment with it and see what you think - resale is not a problem with McIntoshes. Good luck! Arthur