My set up is similar to yours in regards to speaker placement and distance and I have a Technics SU-G700M2 as well and I can assure you it’s not the integrated that needs to be replaced.
My listening space is smaller (10.5’ X 14.5" in a "room" that extends another 5’) but alas, no greenery in the corners. That shorter front to back wall is a factor in base reinforcement but I’ve had speakers that no matter what I tried, were a bit weak and thin across the spectrum despited all my efforts to better it. Even my JBL 4319 monitors with their 12" woofers had impact and dynamics but not that fullness I craved.
What made the difference are my Revival Audio Atalanate 3 monitors with "only" a 7 inch mid-woofer. Practically the same base but more fullness, musicality and body with no sacrifice in detail, air and ambience. It’s a match made in heaven for me.
I’m sure some here may tire of my recommending these speakers but they are the last ones I’ll ever get unless I win the lotto and then get a bigger space to accommodate their bigger brother, the Atalante 5.
All the best,
Nonoise