I would also think about demagnatizing/degaussing the head on all those units. Helping to get things back in line and removing magnetic fields that collect along the path of the tape. Works for reel to reel and casette also. I remember we were working on a song to sell to Ricky Skaggs. I listened to the recording on the reel to reel and then used the unit, and felt it sounded better to me. So we mixed it all final and sent it out to be mastered.
The unit has plastic protectors on the tabs and is 2 or 3 inches long. So you can reach inside and keep from scratching the block and heads. You hold it right up near the head, push the botton, and slowly pull it out and begin turning it in a circular motion. Tight little cirlses while leaving the unit, counterclock wise. Little 1/4inch circles to start. increasing the size of the circles until it is 3 feet away and the circle is 2 feet in diameter. Then turn it backwards and release the button.
I don't know where mine is right now. SO MUCH music stuff. Now that many things are in storage, it might take a while. LMK if ya want me to look for it. I can send it out to ya and then you can send it back after use. Or buy yourself a new one. Either way... Just a suggestion......Danny