OK. Have listened to it with a critical ear a few more times, and then played the new stereo mix and recent remaster CD through the same setup.
Dweezil's stereo remix sounds fine in the bass department. If you read his comments in the book, he says his goal was to bring out more bottom end and detail in the high end. If you listen to the stereo CD, it would seem that he has done some of that. But it is not translating to the Atmos mix for some reason.
My CD setup does not use the sub, so that might be part of the issue. It's just playing the music through my left and right speakers with no bass transfer to a sub.
In the Atmos mix (folded down to 5.1), especially compared to the quad, I hear more boominess to the kick drum, and I think that is mostly what is setting off the sub. It's actually tastefully done and not overwhelming, which is contrary to a lot of modern mixes. It's certainly different from the quad mix where the drums are very mid-heavy and the kick is very low in comparison to the rest of the kit, though that was not uncommon for 70s recordings. The drums overall in the Atmos mix sound great, I think. I like where the kick level is, the toms have a lot of punch and detail, and the cymbal crashes don't hurt my ears like they do at times on the quad.
But, it seems to me like the bass and organ, which carry a lot of the balls in Deep Purple, have been dialed back. Whether this was to clear some mud or make room for the kick, I can't tell. To my ear, it seems like they were given a pretty aggressive curve on the low pass filter (high pass filter in this case, I guess)-- like in the low 100s. Either that or they're just a couple less dBs in the mix, but to me it sounds like a frequency thing. The quad mix, in spite of having minimal kick drum, does not have this same issue and it's because it lets the bass and organ carry the low end, while the drums still carry the madness and fury that is so effective in DP.
What I'm curious about also is whether some of the liberties taken on this mix were on the original tape. The closing scream on Highway Star sounds suspiciously like a cut and paste job from the opening scream. I actually like it, but I don't want it there if it wasn't on the original tape and is just artistic license taken by Dweezil. Same with echoes in the choruses of Smoke On The Water. Again, I like it, but where is it coming from?
As for the panning and stuff itself, I have no problems. The mix itself is adventurous and well-balanced, never out of whack. I only have problems with the low-end ON THE ATMOS MIX and any potential liberties he may have taken.