To me it sounds like the impedance and/or inductance of the preamp inputs of the amp changes as it is powering down, especially if the impedance drops that would make the volume level increase on the other sub. Are they truely daisy chained? As in, preamp from reciever to first subs inputs, and preamp output on first sub to input on second sub? If thats the case I would try using a splitter from the reciever and avoid using the preamp outputs on the subs.
