I had the same problem with an MTX thunder 250D. Wouldnt shut off, i checked for stray wires and couldnt find any. If i pulled the fuse from the distro block for a second then put it back it would kill power and would not come back. I was only using the amp as a short term replacement until another amp was fixed. It would also keep my other amps powered up to (since i was jumping the remote wire from amp to amp). But as soon as i cut power to the MTX all the other amps would lose power as well. I never did figure out why, i just assumed it was something with the MXT.