Weak Internal Amp And Oveoad Problem

I used my new pmp6000 at my first gig Sat night. We have 2 guitars, bass, keys, electronic drums & 4 vocals running to the board. It worked okay for the first set, but soon we started getting weird distortion & a muddy sound from the mains. That turned into a crackling like a blown speaker sound and then just kept getting worse. I also noticed that the gains were clipping on almost all the channels. I kept turning the gain structure down to try to reel it in, but this did not help. We ended up cutting the gig short as this board failed and our backup was not with us.The next day I tested it out & couldn't replicate the same issues. However, I noticed that output A was fairly muddy compared to output B. I would pan from one side to the other and could tell a noticeable difference in the clarity of sound. Additionally, the output was not near what I expected. It should be close to 400 watts per channel into 8 ohms. It wasn't close. To prove this, I disconnected the outputs and went from the unpowered main outputs into another amp that is only rated at 300 watts per channel into 8 ohms. It was much cleaner & louder. I then took this a step further and hooked my equipment up into another mixer and i to the same 300 watt/channel amp. The old mixer I had was much cleaner, warmer, more depth, better clarity, better gain structure, more volume at much lower slider & gain levels. And the mixer I was usi.g was just an old Mackie 808M powered mixer.So, what is wrong with my new PMP6000? Why is it not as clean, not as warm, depth, clarity, or have the output it should have? Thanks!