The mapletree preamp has separate PS, so EMI is less likely, transformer is at least 12" from the tubes. Ken uses very well shielded ICs.
Ken, is this the same hum you asked about on AC having to do with the DirecTV or is that a different system? The reason I ask is because even if the DirecTV is shut off, the mere connection of the grounded antenna wire to any part of the system will still give you the hum even if the directv is shut off. It gets in on the ground of single ended interconnects, or on the shield of correctly grounded antennas.
Verify it is not the preamp causing the hum by running it into one poweramp, both plugged into the same outlet, with no sources connected to the preamp input. Disconnect everrything except the single connection to the poweramp. If no hum, then add in the other poweramp, using the normal outlets for each component. If it hums, then try the ground buster on the preamp. If it is quiet, then the preamp is clean, you can start adding sources. If not quiet, lift the ground on one amp and plug the preamp into the same outlet as the grounded amp. If still hums, get a new preamp.. but I don't think it will unless it is EMI.
Add only one source at a time. Disconnect a clean source before testing a different one. If none of them hum, that means the hum is between two of the sources. Start adding them in together one at a time. When you find the hum, start removing the other sources until you narrow down which two make the hum.
In the end you can either lift signal grounds in the preamp or source (surgery,) lift safety ground with ground buster (not recommended,) use floating ground with balanced power transformer, isolate coax antenna with cap or transformer, or isolate analog signal with cap or transformer, or get an iPod.