Amp arcing to sub?

I was troubleshooting a Rockford amp this weekend. It's a Rockford 900something that is bridged to a Kicker 10L5 wired at 4ohms. The story goes that all of a sudden, there was no output. I ran through the basics: making sure the amp was on, checking the voltage at the amp, eliminating/bypassing existing RCA's, checking the signal... all that was fine. So then I tested the output of the amp by using another speaker. When the amp was on but the volume was down, I connected the new speaker and there was an arc from the speaker wire to the amp and the cone of the speaker went to XMax, as it would do if you would "pop" a speaker with a battery. That's not normal that I recall so I grabbed my DMM and tested the voltage on the output of the amp. It was reading 28.2V. Is that normal with the volume down?

I tried to think of anything like this that I have encountered in the past. I know that as volume is increased, the amp's output is increased respectively, not just wattage but voltage too. In one SPL vehicle that I participated in, we had 4 Kicker 15L7's in a Ford Festiva with the biggest Class D amp Kicker makes on each voice coil. To adjust the amps so that they performed the same, we measured the voltage coming out of the speaker outputs on each amp. I have never heard of 28 Volts coming out of an amp with the volume turned down or even up for that matter. Nor have I ever known an amp performing correctly but yet keeping the woofer at XMax continuously. I told the guy that his amp was the problem but I wanted to double check with the forum. Does anyone have any other suggestions?