I am not sure what you have heard and how you define the sound you are looking for. I have heard a pair of ATC's at John's and they sounded sort of boxy to my ears. Not very open or layered so that is what I look for to begin with. But I also like hearing a warm boxy type of sound also so it comes down to the preference you prefer always. When you took your preamp to John's did you compare it to something John had to see if you hear a difference? IMP the preamp is the weakest link but you need to hear and compare for yourself. Upgrading your equipment you may discover that the improvement with your current speakers was what you are looking for. It is hard to know what you are hearing and what sound you are looking for as you have net described that for us. There are plenty of times I have heard something in a system, at a dealer or at a show and was not impressed like you and thought the upgrade was not significant.
That is where understanding what makes sound do what it does and how you prefer components in your system. We have several systems in our listening room and since we repair audio components, we get to hear so many different components in the various systems. Plenty of times we get surprised at what changes the sound of a system. For example, we repaired an older model Art Audio stereo tube amp 20wpc. Tested them on Vandersteen Sevens and the sound was excellent. We never would have thought that 28wpc would work that well on 83.5db speakers. It was better than many top end 100wpc power amplifiers from other well known manufacturers. So you need to understand the sound you are searching for and experiment to find what works for you.
We have also heard many higher end speakers and components and were no that impressed. Like you, we have heard Wilson speakers in one system and thought nothing special. Then we heard them in another system and were impressed with how they sounded. We have also heard various set-ups at reviewers and sometimes we were not impressed but people read their reviews and think they all have golden ears. Go Figure!
This hobby is not hear something once and that is it. Try, try and try again and you will discover new things that you prefer. Then try again! LOL
Happy Listening.