Thanks! I didn't particularly like the bass on the mono tapes I played and I hope this is the cause. The tapes are production masters which contain transfers from 78 sides so it's just as likely be the recording.
Another question about this general subject: is there a reasonable way to determine if a tape labeled "mono" was recorded as full track or two track mono? The tapes I have so far are labeled so it's clear but it seems likely I'll face this issue in the future.
I do have a mono repro head for my deck (Otari MTR10). I need to mount it and to figure out how I want to handle routing the signal from the head. I really don't want to make any non-reversible changes, have an awkward cable dangling, or put a switch in the signal path. I've already spent some time pondering how to proceed and, so far, I've come up empty.