After looking at your system photo, I agree you would likely benefit considerably from trying to do something about your equipment placement. I DON'T think this is the reason you don't like the U.S. amps you have heard, as bad equipment placement is going to more or less effect all brands adversely (including the U.K. ones that sound better to you).
As an experiment, try getting all your equipment out from in between your speakers and off to the side (on top of a support beam in the flooring, if possible). I know a lot of people use center-stage, symmetrical positioning between speakers for their electronics (allows for shorter speaker cable runs, etc), but in my room/system I was amazed by how this degraded my sound, even using a very good rack. In this position you subject your audio chain to unnecessary vibration from your speakers. This is particularly noticeable if your equipment is resting on wood flooring in an older or less sturdily constructed house (concrete flooring in basements is probably less of an issue). For similar reasons, Sean's point about the subwoofer firing at short range into your equipment is a worthy one. Try moving it.
These are simple things that may well end up giving you more sonic improvements than several thousand pounds in better gear (no matter what its national origin). They are worth a try.
As an experiment, try getting all your equipment out from in between your speakers and off to the side (on top of a support beam in the flooring, if possible). I know a lot of people use center-stage, symmetrical positioning between speakers for their electronics (allows for shorter speaker cable runs, etc), but in my room/system I was amazed by how this degraded my sound, even using a very good rack. In this position you subject your audio chain to unnecessary vibration from your speakers. This is particularly noticeable if your equipment is resting on wood flooring in an older or less sturdily constructed house (concrete flooring in basements is probably less of an issue). For similar reasons, Sean's point about the subwoofer firing at short range into your equipment is a worthy one. Try moving it.
These are simple things that may well end up giving you more sonic improvements than several thousand pounds in better gear (no matter what its national origin). They are worth a try.