Here is my set up:
Gibson LP 1960 Classic
Mesa-boogie Dual Rectifier
Bogner 2×12
DMBTT
George L’s cabling

I’m running it in the effects loop with no other effects. I have these issues wether I run it serial or parallel, but overall guitar tone is less tainted in parallel mode.

Series loop 50%:
With Loop mix @ 100% & the delay mix @ 20-50% – the volume is matched with the loop on or off, as long as the effect is off. When I turn on the loop & the effect, the volume drops at least a few db & tone is very tainted. I noticed that the dry tone with the effect off & the loop on is degraded considerably.

Parallel loop:
Loop 10-50% & delay mix 100% – volume drop no matter where the gain is, but the higher the gain the less the volume drop. However, the higher the gain the worse the delay sounds. It gets quite unusable.

I believe this amp has a tube buffered effects loop. Is it that the issue? If so is there any adjustment or fix? If not would a DMMTT solve this?

I should add that the tone issue occurs @ any delay time. Obviously slower repeats provide more analog mush factor.