From the beginning of these units being manufactured, they have changed. I have owned 3 different models and wouldn't recommend 2 of them to anybody. Funny as it seems their first unit the P300 was the best by far of the bunch. With the beginning of these units, AC output was balanced. It made good sense, but not for every unit out there. After the initial line was replaced with new units, the balanced idea went out the window.
Next week, I will be taking ownership of a P300 which was upgraded by an audio engineer who made a few changes, and of course, changed every electrolytic capacitor in the unit. Seems like he was using different output transistors too, but I don't recall.
I am a real fan of the P300, but not of the P500 nor the PPP. Nothing but trouble. FWIW, I read about a LOT of trouble with some of the newer models as well.