New hum in preamp power supple and BPT 3.5 ???

You have transformer hum right? If so, sounds like it could be "DC Offset". This will probably not be helped by AC ground changes. (which is called a "ground loop hum").

Here are some of the possible causes....

A. Too high AC mains voltage for the primary
B. DC offset in the AC mains
C. Too high a current draw from the secondary
D. Core not in proper electrical contact with the ground (i.e. ungrounded core)

If this is your problem, the best fix is an issoliation transformer/power regenerator/balanced power/ type device.
Some brands are Powervar, Blue Circle Music Ring, PS Audio Power Plant, Equi=tech, and AudiophileAPS...

While fixing the transformer hum in your gear; in some cases the transformers in the AC power device will hum for the same reason the gear had a hum. LOL!

Are you using dimmer switches on the same circuit or fluorescent lights? Not sure if they'd be a cause, but if you have them on that circuit, get rid of them anyway.

Sugarbrie, the BPT 3.5 is a balanced power line conditioner. There are dimmer switches but the hum exists with those lights on or off.

Oh..rats... My last comment then means it might be hard to fix. The same thing that the conditioner fixes (ie, the gear transformers won't hum); unfortunitely the conditioner's transformers are getting the same "off" power from your neighborhood power grid.

Did any of the gear transformers hum before getting the BPT 3.5? Let's hope it is a temporary problem (like your utility doing maintenance) and it will go away.

You could get a dedicated line if you don't have it already. Can't hurt. Very last resort is put the BPT in another room and use a very high end power strip with a long extension power cord?

Or try another brand balanced power device....

I had a similar problem. In fact, my amplifiers (Levinson 436's), sub-woofer, and BP-3.5 were all humming. So, I installed dedicated properly grounded 20 amp circuits with Hospital Grade Outlets. Problem solved.
I think the BP-3.5 likes to be fed 20 amps. So did the amps and Sub.
Now, everything is stone silent and sounding great, night or day.

The circuit I am using now is 15 amps. I have been meaning to install dedicated lines. This problem inspires me to do them now. Thanks for your help, guys.

Brooks1 could very well be right on the money. The tranformer hum is a problem with the electrical current, so give the new 20amp lines a try. Good thing to have anyway...