Pearl 2 and SUT “suitability”

Last week a buddy lent me a SUT to try. I use a Denon DL103. Connected it to the Pearl. A lot of hum. Then proceeded to lower the gain in the Pearl by means of altering R14 if I recall correctly, less hum but still too much noise. The SUT had some input variables that I tried without a difference.
BTW, I could tell that there was no noticeable difference by using it.
My question is, out of curiosity, is the Pearl2 not compatible with a SUT?