I had a popping issue with my A5 that was due to a failed op-amp module. However, it got worse over time, and was pretty easy to reproduce (basically every time it turned on). That does not sound like what you have, though.
Rather than postulate that amps sound different, you would be better served, I think, asking what types of distortion are audible (and under what circumstances).
As a consumer, you don't know if it meets local regulations and standards ( regardless of whether the actual certification has been done). Much easier to bypass like this than with something that has a internal PSU. Point being, anything above 50 V (maybe 30 VAC/60 VDC depending) would not be...
Obviously state of the art performance, but at this level of performance and price point, I hate that it has an external PSU. Especially as it is above 50 V, which is the threshold for many electrical safety rules in various places.
Do you mind also trying the infrared temp measurement again, but with a piece of masking tape on the surface being scanned? Either beige or green will likely work, I'm just curious about the effect of surface emissivity. Masking tape is supposed to be pretty close to 0.95, while anodized...