The mains are fine, but I can tell you that when the cord (not assembled by me) was reversed on line and common there was a noise from grounding through amplifier.
IMHO it is the first suspect. I'd check whether all lines are OK in mains outlet first hand, there is clear connection in the power cord and the power input socket in the Bugle wired correctly.
Second consideration: you have grounds from all switched RCAs input + output RCAs connected together. To prevent mess from other inputs - check the noise when all inputs are disconnected. As far as I understand - the noise is here always, without relations what input is selected?
My volume is in another box, all connectins are floating, but if the input and output are switched (RCA's going to it) it works but then touching the knob increases a ground loop noise it gets in reverse.
Is the "another box" with the volume pot is shielded/grounded in any way? The symptoms you described are like the problem in the ground connection through the interconnect. Jim had already mentioned the integrity of the interconnects, but maybe something wrong with Bugle's or preamp's side of one (or two) ground connections? It's easily to spot. Disconnect the cable (interconnect) from the preamp and check the continuity of both outer contacts of RCA plugs to corresponding input RCA shells in the Bugle.
Other tests: battery supply of the Bugle, shorting out the output RCAs temporarily, pulling out opamps (one at a time, beginning from the last)...
