Preamp Noise with High-Efficiency Speakers

This could be caused by a ground loop. A ground loop can exist if your equipment is built without a proper grounding scheme.

If you float the ground pin of the amplifier power cord from the wall (using a hardware store ground cheater) and the buzz is solved then it is the amp. If this works best on the preamp than the preamp could be at fault. In either case it is not recommended to operate the system without the equipment being properly grounded- there is the risk of shock or fire hazard if a component develops a fault.

If you are able to sort out which component is the culprit, you will have to come up with a solution. The best solution is to have a proper grounding scheme installed in the defective equipment. If the manufacturer gives you pushback on this have them give me a call (seriously- setting things up right is not that hard).

Otherwise an isolation transformer for the problematic unit could sort things out.

Thank you all for the very insightful suggestions. I'll do some experiment early next month. I will try removing the ground pin (on power cord) one by one, and then try a passive preamp to see if it's a gain issue. I've had a Benchmark DAC-1, even at 0DB setting (10 volt output on XLR!) it is dead quiet with the same pre/amp. Will report later.

I transitioned from electrostats to the Avantgarde Duos almost seven years
ago. Quieting the system was a major effort, but yielded enormous
rewards. I agree with the above remarks about trying to find the source of
the hum. At a certain point, mine was not pronounced, but I wanted the
system to be dead quiet and worked further on the following things, some
of which you have done:
1. dedicated AC and subpanel
2. running a noisy compressor for my tonearm into an isolation transformer
3. identifying noise inducing appliances that contribute to low level hum on
the system- even dedicated lines don't really 'isolate' the system power
from the rest of the household electrical system. For example, a hi-
intensity, low voltage spotlight over a kitchen sink 3 floors away will induce
hum. Solution- when I am listening, we don't turn on that light. Ditto on a
room humidifier in one of the bedrooms. Finding the source may thus not
even be 'in the system' in my experience.
4. Cable and equipment layout, as others have mentioned.
5. Playing with grounding solutions- for a while, I used that Granite Audio
Ground Zero- a sort of external star grounding system that allowed you to
change impedence on various grounds (these are not a substitute for the
ground to power receptacle but in addition to it). It worked for some system
set-ups, but I'm now at a point in my evolution where I don't need it.
6. Others with more technical experience can weigh in here, but as I
understand it, different components may have different internal grounding
set-ups. That may contribute to the problem.
7. Alot of this is simply time consuming and mind-numbing, frustrating
experimentation and fiddling.
8. Positive note: it can be done! My system is extremely quiet. And the
results are worth it, not just for avoidance of hum, but to yield more
information coming through the system.
9. Anecodotal observation: The Avantgarde Duo is tricky in the extreme to
really nail. A lot of folks critical of it have heard it set up badly. I'm not
suggesting that it is the 'best' or 'better than' anything, but I can get an
extraordinary amount of music out of my system.

I too have super sensitive Avantgardes (107 dB Duo Omegas) and run a tube pre and tube amps (as well as an AMR tube CD player, and analog with tube phono stage) with virtually zero noise - one must put their ear right up to the tweeter horn to hear the slightest hint of tube rush with no music playing, even at "generous" volume settings.

I would definitely check for ground loops first, and ensure that you have clean power (I run dedicated 20A circuit for my amps/sub amps and dedicated 15A circuit for my pre and sources, the latter through a Shunyata Hydra).

A tube pre with separate L&R gain controls will allow you to dial in the sweet spot of gain for your rig, as well as providing a de facto balance control, handy in asymmetrical rooms.

What's wrong with just using the AMR DP-777 with it's volume control or just use a passive preamp like Lightspeed Attenuator or similar with just a pot in a box, no noise with passive pre's. And as you have such high sensitive speakers you are throwing money away using an active preamp, unless you like the colouration they can give.

Cheers George