Hi, I was wondering if someone could help me with some info about this amp. It has been explained to me that they changed a circuit at one point. After the initial production. That made it a lot better. Could you guys tell me what serial numbers correspond to that change? I've been wanting one of these amps for some time now. And I want to get the correct one when I finally pull the trigger. I usually only see them for sale online so I may not get to test it or pull a tube to check it in person before purchasing. Thanks in advance.