Just picked up a 66 non reverb Princeton in near mint condition. It sounds very good too. I have a buddy with a 68' Silverface PR who got the phase inverter mod and he said it made his amp almost as loud as a Deluxe. I would love for mine to be louder but it's so clean and completely original I hate to mess with it or devalue the amp. I realize the PI mod is reverseable but I'm thinking it would still lower the value?