Hovland pre-amp susceptible to A/C line noise?

I have not yet been able to talk to Hovland about the following problem and they have thus not yet had a chance to try to resolve it, but I just purchased a Hovland preamp over the weekend and am having a big noise problem. The preamp seems incredibly sensitive to A/C line noise and thus far has been unlistenable as a result. I floated the ground with a cheater plug to kill a bad ground loop problem (this worked), but I still have a strong and clearly audible buzz/humm. I tried the preamp on two other outlets, and tried putting all source components on different outlets too (and used cheater plugs on everything), but the buzz/humm remains strong (preamp is basically unlistenable). I have both a halogen lamp and dimmer lights which greatly increase noise level (but these lights have never bothered my other components), but turning them off still leaves me with way too much buzz/humm. The only time noise level is anywhere near acceptable is when everything in apartment is turned off (including all source components!), except for amp (VAC 70/70) and Hovland preamp. I tried a Chang power filter last night, which yielded no improvement whatsoever. Getting ready to try Richard Gray filter (and if that doesn't work, maybe PS Audio filter). I've had many tube and solid-state components in this apartment (including two tube preamps, tubed DAC and tube amp) and never before had any noise problems other than ground loop problem.

Any ideas??

Again, in fairness to Hovland, they have not yet had a chance to address this.

Thanks.