In the tapeheads.net (sadly gone) forum, it was mentioned that playback of analog magnetic tape recordings need about a 2dB maximum smooth treble boost to restore their fidelity and to make Dolby A NR tracking work properly, I'm fine with that since it's a restoration technique which doesn't cross over to enhancement.
On the harshness issue, maybe the Captain and Me quad mix was near the audible harshness tipping point (they wouldn't know about such things ~50 years ago) and boosting the treble for the BR mix pushed it over the tipping point into audibility.
I've read some about LP mastering, basically trying to fix the mix w/EQ and other signal processing between the master tape playback and the LP mastering system (CD-4 mastering put an end to that, listening to the sum/baseband signal only at 1/2 speed made it impossible to fiddle with the quad mix).
(it seems simpler to make the master tape sound exactly the way the artists/producers want and then try to transfer that fidelity to an LP unaltered)
Kirk Bayne