From the RR2150 manual, "The external processor loop is a "fixed level" loop. It is not affected by the RR 2150's Master Volume Control." You'd need to enable the External Loop on the front panel and you'd need to have the desired source active (unlike the tape loop, which gets whatever source is selected at the "Record" knob on the front panel), but it should work - assuming you don't need the external processor loop for something else...