Your dealer is probably a moron, and probably desperate to hit your wallet after throwing you in a rabbit hole.
It looks like you have some good gear. Why do you even need a new dac? What’s wrong with your dac? Looks to me like you don’t need a new dac.
The only issue seems to be that your single sub is most probably in the wrong location and very poorly integrated.If you have 24ft, speakers are say 4 or 5 ft from the wall and you are sitting 10 ft away from speakers, you have ample space to put that sub behind you....That’s where it most probably should be. Sub placed up for aesthetic reasons is most probably a fail.
Do you have a mic/REW? Ever measure anything?
You could also put something diffusive at the ipsilateral/contralateral reflection points.