I had the same problem, though I had to lift the ground on both amps to get it to quiet down enough. Great question. Don't have any answers, but I'm very interested in someone who does. Thanks, Greysquirrel
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The basic problem is that the interconnects ground the preamp to the power amp. If the individual chassis are then grounded via their power cords you have a ground loop. Even if you use the same AC outlet for all power cords you may still create a loop. Since the preamp is the low-level signal amplifier, grounding the preamp and floating everything else gives the lowest hum and buzz in most systems. The preamp is also the most rf sensitive component. So, since the preamp is grounded to earth, and the amps are grounded to it via the interconnects, rf tends to move toward the preamp chassis and bleed-off to earth. Besides hum, buzz is created by down-conversion of AC line harmonics via the parasitic capacitance between your power transformer windings. This is something that the new PS Audio power conditioners address. Ditto for the Jensen input transformers.
I run tube monoblocks with a dedicated star ground from each with the star meeting at the ground of my preamp. I have Two PS Audio Power plants that also ground to that point which is 8 gauge wire to two 16 foot copper rods dirven ito ground right outside the listening room. The Monoblocks are on a dedicated 240VAC circuit split 120-120 to provide "balanced" AC two them. There is a second dedicated 240VAC cicuit split 120-120 for the two PS audio units. The grounds at the plugs are lifted from the AC mains ground and connected to as legs of points of the star ground. There have been lengthy discussions on this topic under the headings of dedicated lines and power conditioners. Check them out for some great info. The dedicated earth ground and the PS audio units killed all the noise on my truely terrible utility power lines.