It appears that you are describing a lack of dynamic contrast. This could be dynamic compression. Or, phase cancellation (the room). Or, both.
Right off hand, it looks like there may be a power strip with lots of stuff plugged into it. You might try plugging the power amp directly into the wall to see if that helps. The unknown source makes it difficult to zero in on the problem/remedy.
It could also be that everything is "okay", but the speakers lack the dynamic impact that you are looking for? 87 db efficiency isn’t "low", but there are high efficiency speakers out there that require 1/10th the power to drive that will push the guitars and vocals more out front, compared to the Thiels.
A good evaluation "tool" for room acoustics is to set the preamp at a fairly low volume level and set very close to one of the speakers. Does it sound better?
While this system may not make the cover of Stereophile based on its aesthetics, I’ve seen much worse. We had an amp come in to the shop for repair recently, that when the cover was removed .... I’ll just say I’ve seen cleaner Porta Potties at outdoor music festivals.