It is an interesting request.
Intellectually, it doesn't make sense to me, in that Mike recorded his masters with Dolby on and made his copies with Dolby on. Transferring with Dolby off would leave intact an artificial high-end boost (emphasis) that is not accurate to what he captured, in that Dolby decoding would tame (de-emphasis), which is how the Dolby system works in a nutshell (see longer explanation below from Wikipedia).
Emotionally, I get it, as you are not alone in thinking things sound better with Dolby off.
I have seen posts where people claim Mike transferred with Dolby off, but I have personally seen perhaps 60-80 first gen tapes Mike made for other people. EVERY SINGLE ONE OF THEM is marked Dolby B on in his handwriting. If he was an advocate for Dolby off, why record with it in the first place, why give out copies with it on?
For me, I think the best path (as Winston has followed) is to master our Dolby on transfers, and bring back some of those highs. But I would point out, the sound you prefer in the older versions and assign to Dolby off may also be caused by mastering someone did, deck alignment, chrome/normal switching, etc.
I am working on a lot of other Millard tapes at the moment, many new to circulation or previously un-attributed to him, so I can't really prioritize doing the LZ tapes again (which were borrowed and would have to be reacquired though I may have access to a different set at some point soon). Any change would be on the margins regardless and there are already so many versions floating around, not sure doing so helps matters as it is all personal preference. At least the transfers we did were unique in presenting the recordings as he intended. The flat masters, so to speak.
I've probably transferred 50+ Millard-made first gens at this point, and I can only think of two where I thought Dolby On was the wrong setting (perhaps he forgot it when he dubbed but noted it as on).
In other news: Given this is a Stones forum I will let you know some Millard '89 Stones tapes will be coming down the road as well as Mike's own personal best-of compilation from LA '75.
Dolby noise reduction is a form of dynamic preemphasis employed during recording, plus a form of dynamic deemphasis used during playback, that work in tandem to improve the signal-to-noise ratio. While Dolby A operates across the whole spectrum, the other systems specifically emphasize the audible frequency range where background tape hiss, an artifact of the recording process that is similar to white noise, is most noticeable (usually above 1 kHz, or two octaves above Middle C).
The Dolby preemphasis boosts the recorded level of the quieter audio signal at these higher frequencies during recording, effectively compressing the dynamic range of that portion of the signal, so that quieter sounds above 1 kHz receive a proportionally greater boost. As the tape is recorded, the relative amplitude of the signal above 1 kHz is used to determine how much pre-emphasis to apply - a low-level signal is boosted by 10 dB (Dolby
or 20 dB (Dolby C). As the signal rises in amplitude, less and less pre-emphasis is applied until at the "Dolby level" (0 VU), no signal modification is performed.
The sound is thus recorded at a higher overall level on the tape relative to the tape's overall noise level, requiring the tape formulation to preserve this specially recorded signal without distortion. On playback, the opposite process is applied (deemphasis), based on the relative signal component above 1 kHz. Thus as this portion of the signal decreases in amplitude, the higher frequencies are progressively more sharply attenuated, which also filters out the constant background noise on the tape when and where it would be most noticeable.