Improve sound

synergy is a bitch. I would try smaller, more musical speakers first that you can return. (Revel, Buchardt, Dynaudio, Totem, Q acoustics, Monitor Audio, ....)

I think your DAC is fine, the least problematic. Your amp is OK, but does it work with your speakers? Start with a small change, bookshelf speakers. Look for music that you love and listen and look for sounds that blow your mind and make you want to turn up the music. You need a baseline, something to strive for and it seems you have been feeling "meh". 

I know nothing about your amp but it's safe to say, it's for home theater/entertainment and not for peak audiophile joy. 

The short version is: you have to learn about yourself. Neither your speakers nor your amp is audiophile, maybe midfi, if you sell both and add $500 you will have a lot of options ($2500?)

good news!