Let the record show, Les asked for it! I reduced the value of the coupling caps going into the phase inverter, to reduce the amount of just the very lowest frequencies on both channels. I also increased the value of the tone slope resistors on both channels, basically to shift more of the signal flow to the treble control, so I didn't have to run the knobs at extremes all the time (max treble, minimum bass), and it makes the amp less picky about "sweet spots" on the controls. That last part could be in my head though. It did definitely shift the EQ mildly, which was all I needed. The presence knob has a "pull deep" function on it, which is basically a negative feedback resonance control. Even in the normal/in position, there's still some effect to the tone - I would call it adding low, but I think what it's actually doing is removing some high which gives the illusion of adding low - anyways, I removed the capacitor that was doing that, so normal is now unaffected, and I can still do pull deep. The two barely had any contrast before, and now it's more noticeable, and more useful IMO in the "normal" position. With the deep cab, I really have hardly any need for the added depth setting, but more on that later. My last mod, as you can see from the photo, was to break the presence control knob off the pot shaft. Apparently I screwed the little hex set screw in too much and the thing cracked wide open on one side. That was surprising, as I wasn't torquing on it much when it happened. I have some superglue and have been meaning to glue it back together, it was a clean break so it should be fine. I haven't found a vendor that sells replacements, so if that doesn't work I'll call PRS. I also learned why the presence control itself is so touchy on this amp. It only changes the tone between about 8/10 and 10/10. I thought maybe I had a bad pot, or something had been wired incorrectly. What I discovered is that the pot is bridged with a second, fixed resistor in parallel with the pot. Many amps do this on their presence control as a "voltage divider," long story short it's usually done to reduce a scratchy pot sound on the presence control. The reason this amp has such sweet spot between 8 and 10 is because the pot itself is a 50K pot, bridged on the outer lugs with a 4.7K resistor. So, basically, that's a small fixed resistor in parallel with a large variable resistor. If you're familiar with how resistance combines in a parallel circuit, path of least resistance, etc., you can imagine how a 50K pot has to be turned to the point where it only has a resistance of 4.7K for there to start being a considerable overall effect on the resistance of the circuit. Which, if we were talking about a linear pot, would be in the last 10% of travel - whether this pot is linear or not, I'm not sure, but that 10% figure doesn't seem too far off when I fiddle with the knob. I'm considering tweaking that circuit to get a better sweep, but I haven't had time to do the math and see if I could just swap a resistor. I would assume that later revisions of the Custom amps have changed that, because the control is very hard to adjust with such a narrow sweep of useable settings. I should add that everything above is what I think I did to the amp. I'm fairly confident that I identified the correct parts to replace, and each change certainly had the desired effect. It's a heck of a lot easier finding the right component on an amp with hand wiring, rather than a PCB, but even so this is a complex amp just by the nature of the two channels. Fortunately the circuitry of each preamp channel as well as the depth/presence controls are heavily influenced by traditional M and F circuits, so they were easy to find relevant info on. And, going back to that thing about the deep tone of the big mouth cab, my LAST big mod I have coming up is going to be building a new 2x12! I've got the wood sitting in the garage at the moment, just trying to decide on a design. I've narrowed it down to either a Marshall 1936, or a modified Mesa design, either based on the horizontal or vertical slant 2x12 recto cabs, but not as deep front to back, to cut down on some low end. The big mouth PRS cab I have now is somewhere between a Mesa horizontal 2x12 and an Orange PPC 2x12, which are some of the largest on the market. The Marshall also has lots of internal volume, but every time I play through a Marshall cab it sounds more focused and less bassy. I've heard that it could be due to the method of baffle attachment. On the other hand, the Mesa cab would fit in my truck bed better due to the shorter vertical dimension. If you fellas have any recommendations, I'm all ears. When it's done, I've got a set of V30's sitting in a box, although the allure of Celestion Cream is a real thing too, if the cab comes out as nice as I would like, it might be worth the investment.