BODNC - nice to have another vintage gearhead here!
The preamp section alone doesn't set the voicing. Only if it is preamped out which none were stock. For the total voice you need to account for the whole amp,
From all I know the diffs are on the BMR:
Smaller OT, Tube Rect, 1/2 poweramp tubes, slightly diff tone stack.
All of these combined make for a very different sounding amp.
I own a 74 BRM and a 68 DSR and A/B'd them with the same cabs. Each has a rare and different sound.
The sag from the tube rect & the smaller OT give it much less headroom and a different feel and diff voice.
My high price was for the 68 with drip edge