It must be a setup issue, especially since you say you had to keep the EP175 cranked to max volume at all times.

That should never be the case in a properly calibrated system.

As others have suggested, try the Oppo directly to the sub.

Epic 80 / SVS PB13 Ultra
Denon 3805 / M2200 Outlaw Monos /
Sammy 55" LED