Demagnetization should only be needed after 25 to 30 hours of play, with sometimes after 50 depending on the type of tapes used. Whether it is cassette or R2R, all players will require demagnetizing. Demagnetizing more than that will most likely have the opposite effect and cause you to magnetize your heads.
The tape that has the distortion may not have been caused by you but rather it may have been an "in the field" user error of clipping the mic preamp, thus causing distortion. Most tapers have been guilty of this at some point. The taper might not have padded down the signal and clipped the heck out of his preamp. One way to tell is if he has the encore on the first tape. If that sounds clean, as well as the second set, then in all likelihood it was an "in the field" error.
Those are some great years to have and much needed, especially if it is with something other than Nakamichi 100 or 300 and if the were FOB. Even Senn. 421 or 441, would be preferred. Of course Nakamichi 700 or 1000 were also preferred. That is not to say that Tony S. didn't pull some great 300 tapes back in the day. He had it figured out and some of his 300's are the best recordings using these mics that i have heard.
BTW, I had a friend who recorded with Nuemann KM84's, however, because he sent a too hot signal to his D5 he has hundreds of hours of recordings that are distorted and these were years from 1982-1990. I was doing a bulk transfer and was extremely disappointed with the recordings. A clean recording using a cheap mic is always preferred over one that has clipping. I eventually gave up on the transfers due to poor results.