Preamp Noise with High-Efficiency Speakers

Ground loops are most likely to present 50/60hz "LF Hum", a BUZZ as the OP says he has indicates higher frequency content, this could be rectifier noise, but from 4 separate preamps tube and SS this is not likely.

If the "buzz" raises and lowers with volume control posistion on the 4 preamps, would mean that is eminating from the source. But because the source is dead quite when it is direct to the amp, the source can be ruled out.

If it is a hum and not a buzz you could try this:
If all 4 preamps have grounded mains cable you could try to disconect the earth wire and see if that makes a difference, or if they have no earth wire on the mains try to give it one to the chassis.

Cheers George

Actually if a ground loop is present it will more likely manifest as a buzz. It is rare to see a ground loop only produce 60Hz!

After almost a month the buzz remains with any preamp, and luckily I can still enjoy music running source-direct. The buzz is audible both above and below 140 Hz, I know this because my Avantgarde Duo Classic speakers have active sub crossed over around 140 Hz, and I hear the buzz from both midrange horns AND the sub. I may be able to hear a little hizz close up to the tweeter horn, but it's much less intrusive, and "normal" for almost any speakers. The buzz is independent of the volume setting on the preamp, so it's not source-related. Tuning preamp volume all the way down doesn't diminish the buzz, turning preamp volume all the way up does add a little more noise, but when I listen I rarely go beyond 10 O'clock. I should mention that I did take some wires to connect the chasis of the pre and power amp (common grounding), nothing improves. I did not buy a Ground Zero.

Yingtonggao, did you try using a ground cheater on the power cord of the preamp? If it works it is not a solution, but an indicator.

Ok you tried to give all 4 preamps a earth with no joy, but you have not said if the mains lead on them is earthed?, if so disconect it, if this still is not a fix, "luckily you can still enjoy music running source-direct." which in your case is a good impedance match and you still have an abundace of gain.

Cheers George