I have an ASR-10 and have sampled from every source possible.
I assume you are sampling from the mixer. Could be that your mixer has hiss (or air) which could be corrected with proper EQing.
Sometimes when I get some unwanted air, I just single that frequency out with and EQ. The ASR-10's Paramtretic EQ is pretty decent. SEt a narrow trim level and bring the hiss down, then resample the loop.