Hey gindaddy, the 2 screws that mount the transformer (well (pedantically) they are probably bolts) have their nuts inside the case, so pop the lid when you tighten them so you can ensure the bolts don't spin as you tighten. If this doesn't fix it (a "little" hum is probably normal but in a rack you shouldn't notice it), then put in an MDRT, not only are the MDRTs quiet, they really open the unit up dynamically in a very nice way.
The loop is after the eq (as it should be (for a bunch of reasons)). The loop send is always active and can be used as a mono out (useful if you are using a mixer of some sort (as some of us do) and you need another Fx/aux send that the mixer doesn't have (eg line mixer with only 1 stereo Fx send). The loop return is toggled on and off (in/out) via the patch setting, it doesn't affect the send. Remember the loop level control is a dual ganged opposing pot (turning send up turns return down and visa versa).