I think that the key is the answer to your last question "So many things to consider don't you think?"
Actually - there are TOO MANY things to consider
You may be able to measure amps
you may be able to measure speakers
you may be able to measure head units and so on
But the truth is that you don't listen to amps, you don't listen to speakers and you don't listen to head units
You are listening to a whole sound system and as well - everything around it!
You're probably right on that one but to know for sure, we need measurements
And the volume knob - if it is digital, then it is hard to explain such difference but if it is analog volume/gain knob - then it can be easily explained, very simple, it is not percise...
Volume knobs can rust or collect dust which can hurt their ability to be a conductor or visa versa short circuit it. so many times just by clearing volume knobs (gain more to be true) had solved a hiss (SNR) issue in amps.
IF you payed so much for a milbert and by replacing it with a Sinfoni you started to hear things you never heard before, to be honest - I would ask of milbert for a refund
Never once said that the Sinfoni was better than the Milbert... I simply stated that there was a difference in sonic signature between the 2 amps - even after gain matching...
I can understand small differences but sounds you never heard before??? I can only guess that A class amplifier can be very unlinear but if it is by that much, what is the point of that? and especially in a Car when a sound system becomes "chaos theory" instead of "exact science"
From my understanding, the Milbert is actually a lot closer to being a class B amp than the Sinfoni - high bias towards class A with idling @ 5A+
Still I do think that an amp is mainly an amp though once you implement it in a sound system, it is not just an amp, it is a product which affects and affected by the whole chain and as I said before, choose wisely, a good amp could be an amp for life - thus justifying in any way the price you payed for it.
Don't remember it too well but I think Sinfoni has a lifetime warranty or something like that... There are also other amps where you pay big money and get huge support if something fails even 10 years since the date of purchase... Hard to swallow the big price tag first but it really is worth it in the end if you plan on never selling the amp
I can understand the need to change from 1 amp to another if you just buy from the "middle of the pack" type of amp (Mosconi AS, Helix Comp, DLS A, Audison LRx & Voce, etc...) It's always nice to have the next best thing or the forum boner...
But the questions that interests me more are the "point of reference" related questions -
If you did not know that the amps were replaced? would you still have noticed? My girlfriend surely knew I change something as soon as we drove off (her driving) - asking why is the stage deeper (first from a DLS A2 --> Milbert) and the second time why is the system sounding more dynamic (from Milbert --> Prestigio)
If you bring your system to where you want it (tuning wise) - live with it for a couple of years without touching its tuning (like I did with my first system) then change only 1 thing (Audison LRx --> Milbert) then yes, I believe you can hear a difference even with gain matching...
Better yet - If you let your self get used to one setup and drive with it for a while, after how long would you "forget" how the previous setup sounded like? probably after half a day you would loose that point of reference...