As per my signature, my current stereo system has a B&K Reference 50 Pre/Pro feeding two amplifiers that I built myself. When no source is connected to the amplifiers, there is no discernible noise through the speakers (maybe a tiny bit on the tweeters, if your ear is just about touching them, but not enough for concern). However, when connected (by RCA interconnects) to the B&K, there is a low-frequency hum that is audible from a few feet away. For this reason, I suspect it is a ground loop issue that I didn't address while building them rather than some kind of interference generated within the chassis.
Furthermore, when sourced by my laptop fed through a USB-powered DAC (i.e. no secondary grounding aside from the amplifier's own), there is no perceptible hum - there is just a bit of high-frequency "hiss" that could probably be attributed to the quality of the Nuforce uDAC feeding into a high-gain amplifier directly.
What I'm asking is, is there any simple way to eliminate or reduce this humming? I can't even really think of where to start, aside from putting a resistor in series with the ground path inside the amplifier to cut down on the ground loop current. I just bought a Yaqin CD3 tube buffer today which has an output impedance lower than that of the B&K, so maybe that will help as well.
If anybody could help, it'd be much appreciated. I very much enjoy the amps, I'd just like to eliminate the hum.