I suppose I have a bit of history with the RAT, I've used it since my first band, on various key songs, and have owned several throughout the years. At this point I have a Vintage Rat, Turbo Rat, and You Dirty Rat, the first two have been modified.
The first, easy enough mod is to replace whatever op amp you've got with an LM308 if yours is a newer version. The LM308 has a slower slew rate and a generally darker, nastier sound than the newer versions. It is the original spec'd op amp from the vintage Rats, which was changed out for something else in the 2000s.
The second, fun mod is to add clipping options. In both my Turbo Rat and my Vintage Rat I have a DPDT switch with two antiparallel clipping diodes selectable on the toggle, either LED or 1N914. This gives you higher or lower headroom, which really helps with high output or lower output guitars, sort of like the Fulltone OCD's hi/lo switch. In the case of the Turbo rat, it gives you the standard turbo mode (5MM LEDs) and the Rat 2 mode (small signal silicone diodes). My review of the Turbo Rat is that 5MM LED clipping is a bit heavy and severe for most of my uses, so I find it a lot more useful with the 1N914 mode, which puts it very much in the Rat 2 kind of sound camp. The 5MM LED has more than twice the forward voltage of the 1N4148/1N914 diodes.
These pedals are all very closely related, with the main differences being the clipping diode selection. The Vintage Rat does have more of a smooth compressed midrange character, where the Rat 2 and Turbo Rat have more of an open, clear sound that's nice as well. These differences occur in the circuitry that is not the clipping diodes, so I'm glad I have both. Just depends on my mood which I want to use more.
There is a new Rat out called the Fat Rat with a MOSFET clipping option I think, I have not tried that one yet.
There is also the Ruest Rat mod, to get more bass out of these pedals. I ended up not liking that mod, and removed it. If I need a big bass sound I will use something else entirely. To me, the Rat is all about midrange and cutting clarity. It's that '80s thing, yo.
Other various mods to all of these pedals have been standard Boss style 9-volt jacks (why ProCo, why), adding an LED along with a 3PDT footwsitch to the Vintage Rat, and hammering the steel enclosure.
The enclosures are pretty poorly designed in my opinion. The top surface is too thin, it can be bent by stomping enthusiastically. My turbo rat needed to be bent with pliers and hammered back into a flat shape. The circuit board was actually bending since these pedals are pot/switch mounted on the PCB directly to the enclosure. Bad design choice, the PCB could eventually crack. Also the "fins" of the bottom piece leave a gap between the top where dust and filth can creep inside the pedal from the cracks on the side. I had to fix this with more hammering and bending. I was able to close the gap considerably on my Turbo Rat. My "You Dirty Rat" still has a gap as I have not messed with it yet.
Here's a picture of my Turbo Rat being modified. The DPDT switch was not the best idea, it's very tight in this small enclosure with the PCB mounted. It probably would have been better to use an SPDT for the diode mod. Make sure you use the correct pads on the PCB for the two wires from the clipping mod switch. There are 6 or more pads and you need to pick the ones that aren't connected. I guess this proves they use the same exact PCB for the various Rat models and just mount different sized clipping diodes to the PCB with the various hole spacing. Along with possibly other component variations.
The rat. A nice hangover from the '80s that I'm happy to wake up to.