IMVVVHO (V for very) and based on those I've auditioned, different high-end amps will sound different in different cars with different high-end gear.

And tuning makes the most difference. In the end, the installer's (or the DIY owner's) tune is what you will hear.

Like one time I sat in a car with budget amps (Powerbass ASA600.4) powering not-high-end speakers (Focal 165VR3) with a not-high-end head unit (Eclipse CD7200Mk2). The system was professionally installed, properly wired and RTA-tuned and time-aligned... then re-tuned by ear to sound really good. And it did! It sounded like a system worth 5 times the price. Well-balanced with great focus and staging.

Then I sat in another car with high-end amps (Focal Power Symmetric), high end speakers (Utopia Be Kit No.7) and a high-end head unit (Pioneer ODR RS-D7II with processor). The system had just been installed and only basic crossover settings and amplifier gain (level-matching via oscilloscope) had been done done. No RTA. All EQ set flat. No T/A. Sorry, it sounded bad. Really bad. But its actually what one would expect from an untuned system.

Then the owner of the car brought another amp he wanted to try out. A Zapco REF1000.4 (which is definitely not low-end). The installer swapped it in and adjusted gains. With the previous crossover settings, zero EQ and zero Time Alignment, we were all hard-pressed to hear any difference in sound quality. It sounded equally bad as with the Focal Power Symmetric -which is what we all expected to hear.

That system was since professionally tuned over a 2-3 month period and although I haven't heard it yet, I bet it will beat the ASA600.4-VR3-CD7200Mk2 system hands down in any hardcore SQ competition. I'll also bet novices will hear the big difference with the more expensive system sounding so much better, even to inexperienced ears.

My 2-cents: with high end amps (or high end gear), what one is getting is the POTENTIAL to sound great. Whether or not it will sound great cannot depend solely on the amp (or any one component). It will depend on the installation and TUNING.

Merry Christmas, folks!