SMSL SD-793II Weak on my DT660 80's

Loving the DT660 80 Pro's on their own, but I kept reading they really open up amped. After lots of reading I decided on the SD-793II. I'm not at home to test on my PC, but when using my phone + RCA into the 793 I notice zero audio difference, quality or gain, compared to just running on my phone alone.

Is it possible I got a lemon? Is the amp to weak to provide a gain difference over a phone? Or were my expectations misplaced?