Hi Joe,
If you're using a Dolby Labs 361 (or similar) to decode your "A" encoded tapes and you wish to tweak the levels for each tape (some folks just find a happy medium and live with it), I suggest you modify the detachable front panel and mount a 25kohm input pot. The stock ones (either little blue or black ones) aren't anything special and the 91 series Bournes do just fine (if you wanta get fancy, PEC makes a small 25k pot which is nice but 4x the price).
Just wire the new pots to the interface circuit board (use stranded wire since it may get bent several times and give yourself enough slack so the front panel and cat.22 card can be removed if needed).
That way, you can take the Dolby out of the equation and just pick the best tape machine.