Didn't know that about the preamp stage, but I think you are probably right. I think 32v with 5-6 amps is a good power point for these amps. 32v ok as well.
I think a medical grade supply type PSU is safer to leave on, if you have some sort of casing for it and/or good protection for the wire leads... What is your reason for leaving the amp on all the time? Folks here go back and forth on that issue... I understand the off/on lifespan point of view. As for me, I power a system on in the morning generally and turn off when I am done for the day with it, but I don't leave running all the time. I've had only one brick problem with the D amps, a 24v 5a supply for one of my SMSL SA-50's from years ago - it developed a squealing sound, but didn't overheat, but I retired it nevertheless. The latest power bricks that came from Fosi and Aiyima have presented no problems thus far.
The whole SPL/watts race on these amps is just kind of silly marketing in my view...each of the makers trying to emphasize what they think will push the most units. Same with the op-amp swappability. These are very good little amps for the 50wpc - 75 wpc normal type of listening, for people really wanting to play at 110db they should be spending up and buying a Hyex or Purifi amp, or a modern Class AB amp. They are still good products for most listeners, hype on the specs aside.
I agree with those on the board who continue to point out that the power ratings for them are overstated, but I differ in that I think that does not make them toys or junk. Will they last as long as an old 1970s class AB amp? Probably not, but we don't know. We're likely to get more use out of each amp than the lifespan of our cell phones, at least.