I tried removing a low hum with Cool Edit (now called Audition) and the process took out much of the guitar sound, as mentioned above. Not acceptable. Interested to find a solution since some of those earlier recordings are not replaceable, some done by a departed friend.

BTW, my noise was the result of using a PA interface that wasn't good. After switching to a Dual Pre (preamp meant for this process) to which I connected a condenser mic, and plugging the Dual into the computer's USB port, I eliminated this problem. (Not to say that I haven't found new problems, like latency issues on the multiple tracks...)