Mysterious Hum

Just under 2 weeks ago I switched out my ARC D90B for an SS ARC 150.2. While I always turned the D90B off after use, I'm leaving the 150.2 on 24/7. My preamp is an ARC SP-8 rev 2. Normally I put the preamp's mute "on" before shutting down, a habit from the tubed amp. Today, without thinking, I turned off the preamp while leaving the mute "off" and discovered a fairly loud low frequency hum coming from my Magneplanars. It stopped when I switched the mute switch to "on". So somehow, when I leave the preamp outputs open (even though the preamp power switch is off and the led is out) some signal is making it from the preamp to the amp (which is on) and then to the speakers. But the hum dissappears when the preamp is powered up (I checked this by turning the preamp on, mute off, selector on unused inputs, and there was no hum from the speakers; similarly when I play LPs, on the "interband" tracks, dead silence). Can anyone suggest what is going on, and if there is something I should do?