Luxman leaves me wanting

My current system is a Luxman L590-AXII driving DeVore Gibbon Super Nines. My source is Lumin U1-Mini (w/Sboosterj and Denafrips Venus II. Also a Rega P3. Speaker and interconnect cables are circa late 90s Cardis Cross There is a lot that is right about the sound of my set up, but I feel it’s lacking fullness in the midrange. When I turn on the loudness on the Luxman, I get the meaty weight in the mids I’m missing, but at anything but low volume, it alters the sound in a negative way. 
I had considered a tube Integrated before I got the Luxman, but liked the L590 when I heard it. Now I’m wondering if I should have gone with tubes. (At one point I had a BAT Pre w/ a Classe amp and loved the bloom in the mids and vocals).               I’m attracted to the LTA 40+ from the good reviews and long tube life. I’d be interested in hearing one with my speakers, but there are no dealers, so I could buy and return if I’m not thrilled, but wanted to know anyone has heard the 40+ with Super 9s. Also interested in other options to get a richer sound. Budget is around 8k.