WIIM w/Fosi Monoblocks HELP!

Hi all, I currently have a Fosi BT20A Pro and I wanted to upgrade to a WIIM Pro Plus with Fosi Monoblocks (V3). Well, it sounds awful compared to the BT20A. Like, no bottom end, lifeless. I tried XLRs and RCAs, even bought a Bluetooth unit to bypass the WIIM. Same sound. Why would the monoblocks sound so different than another Fosi amp? Any ideas?? Thanks