If I was to nitpick I would say that the vocals were too buried in the mix. It sounds very much like Jefferson Airplane from which it is descended from, but no Grace Slick. Otherwise a great mix!
I think the lack of more dominant/distinctive vocals is what makes Hot Tuna for me rather forgettable.
Thinking about Jefferson Airplane (on Grunt) makes me wish that "Long John Silver" had also been done in Quad!
A lot of the RCA quad titles that were mixed in-house at their own studios (which is the majority of the early ones) need considerable EQ work to sound "right." I can't remember the exact type of monitoring speakers used in their studios (my memory wants to say "horn loaded driver?") but Steve Hoffman mentioned it when AF's Best of the Guess Who quad SACD came out as he did a lot of work to the stereo mix to make it sound as it should. Effectively, RCA's studios had speakers with a really pronounced top end, so the master tapes (transferred flat) are dull sounding because trebly speakers + dull mix = balanced-sounding end result going in your ear 'oles.
I haven't heard Burgers yet but complaints of 'muffled vocals are a telltale sign of a rolled-off upper-midrange/top end frequency response, which is exactly what I'd expect from an RCA quad master tape of this era transferred flat. So complaints about the vocals being muffled are entirely valid and shouldn't be discounted, and it's worth noting that these are defects of an EQ issue and not a (too low in the) mix issue.
If you listen to how much hiss there is in the intro of the stereo mix of These Eyes on the AF SACD it'll give you some idea of the kind of upper midrange/treble boost that Hoffman had to apply. Similarly, compare the quad mixes of the other tracks on this compilation (aside from the ones sourced from the Share the Land album, which sound decent) which weren't mastered by Hoffman but instead 'transferred by Gus Skinas' - which I think we can infer are basically a flat transfer or close to it - to the equivalent songs on their respective releases from the D-V issues of these albums (which were very much mastered by Mike Dutton) and you can see the difference that getting the EQ right has on making things like lead vocals image properly in the phantom center position.