I did not read every post, but little was said about the room.
I am not impartial on this subject as I used to sell the product, but it would be very interesting to have someone "SigTech" (digital room correction) the room. I performed thousands of room measurements (and corections) and one of the many benefits of digital room correction is the ability to better hear differences in components up stream. Many of the folsk for whom I demonstrated (on their system) the SigTech were sort of where your friend was/is. That is, they were spending gigantic sums of money and not really moving forward --just got "different" sound.
A specific example which was always easy to demonstrate was the difference in a live recording in a SigTeched versus un-SigTeched room.
One of the ways we can tell a recording is live is because we can hear the recording venue --specifically the delayed/reflected sounds in the recording venue. In most rooms (even those passively treated)the recording venue relections are masked (at least to some degree) by the reflections in the listening room.
So, if one of the differences in two components is, for example, air/space around instruments, the room could be masking some of that. Furthermore, depending on what the room does to the frequency repsonse, the room could be masking subtle frequency variations as well.
I can say this: if your friend has not heard his system with digital room correction APPROPRIATELY DONE, (either with the SigTech or the Tact), he has not heard what his system really sounds like. He may not like what he hears (he would be in a tiny minority) but he would at least hear what he has purchased.
As to why the less expensive pre-amp seemed to highlight the differences, I can not comment.
Anyway, further food for thought.