I recently bought a '68 Custom Princeton Reverb, having been charmed at the store. It has not disappointed, and is a delightfully full sounding amp with amazing breakup. Even at acoustic volume levels, it sounds lush. Having a few minutes while waiting for a phone call, I thought I'd plot out the differences in the tone circuits between the standard '65 tone stack and the '68 Custom. The change is very simple, and you may easily convert one to the other. The '65 is in red, the '68 Custom in green. For clarity, this plot is run with the Bass at 3 and the Treble at 7, a common setting. I used the infamous Duncan ToneStack Generator, an ancient Windows app, running in Crossover emulation on a MacBook Pro. The differences are clearly visible and audible: Bear in mind that the low E string is at 80Hz, unless you are Jim Campilongo. In the critical area for midrange warmth (200Hz - 400Hz) the '68 Custom has about 5dB more gain. That's mainly what people are hearing. Treble curves are virtually identical, as expected Yes, the '68 Custom has more bass. Or more accurately, the Bass control doesn't go down as far - it stops at what would be approximately 2 on the stock circuit. Well, that was fun. Now I gotta answer the phone.