If you power down the unit, does the light go out and if so, does it stay our when powered back on (until it comes back on again, I assume).
If the light goes out, try switching the speakers (left to right and reverse) and see if the other channel behaves the same way. If the light doesn't change sides, try the same reverse with the pre. If the light follows any changes, the problem is with the pre or speakers. If it remains on the right channel, you have an amp problem.
If remember correctly, Maggies present a difficult load and that may have caused an amp failure (if the light doesn't change sides when reversing the in/out connections).