Except you're not right.
But lets address it. Even if I were to accept what you say is true the amps you're talking about would be
- Not transparent
- Changing the sound on purpose (and arguably the source intent)
- Measurably different
The point is the general goal most people (and amp manufacturers these days) aspire to is to be completely transparent and to as accurately as possible replicate the intent of the original producer/mastering. As someone else mentioned, the industry has generally moved away from a "signature sound" if it ever really existed at all. In fact many manufacturers now proudly boast that as part of their marketing. Simplistically, what you're basically saying is that certain amp manufacturers are taking a transparent amp and applying eq. If that's what you're wanting to achieve then it's something you can achieve for considerably less.
Back to the reality though, while what you're saying is conventional wisdom, no double blind test has managed to prove it. Ever. There have even been blind tests that tested a $10k+(or was it $100k) boutique amp against a $100 rubbish amp and in a double blind test, no one could tell the difference. I know and appreciate you think you can tell the difference but the truth is, you almost certainly can't as no one else can.
No amount of discussion or evidence is ever going to convince you or anyone else otherwise (if you want more in depth evidence go have a read of my posts on the link I put above) and there's a lot of other reasons to buy a more expensive amp (aesthetics, room eq etc.) but if your only goal is to get a transparent representation of the original producers intent, you can do that for a few hundred £. If you want a specific sound, that can be done with a decent EQ for a few hundred more. It's not magic or mystical despite the expensive amp manufacturers spending millions trying to convince you otherwise.
G