Great score, I'm jealous!
I keep dreaming about a 1959 and a 2203 to finish my already great amp collection; actually it's nonsense as we both already have great amps, but like everyone else we always want more.
I wouldn't mod it, it's perfect the way it is - if you need more gain just boost it with a tube screamer, it's classic Marshall tone as heard in hundreds of great records. The only mod I would ever think about would be adding an extra gain stage because I don't like pedals, I prefer to control it by the volume pot on the guitar and the Plexi doesn't delivers a lot of gain, even when cranked you could use a little more.; but the tone is the best thing ever!
You really need to do a recording demo of this one! Or maybe not, so that I don't get tempted to find one for me. They're really expensive here in Europe nowadays, with the GBP raising and raising against the Euro; also you don't find many great deals on Ebay (from European sellers) and there's no such thing as craiglist here.
Bring on that demo!
Honestly, I did not really like the 2204 I had. Maybe it was just one that didn't have the "mojo," but no matter what I did (OD, parametric EQ, Sonic Stomp, modded DS1), I could not get the sound I wanted or expected out of it. I remember thinking, "These (early 800 series) are the amps everyone puts on a pedestal?!?! My old 1959 sounded way better." So I always wanted to get another 1959 when the time was right. Well the time wasn't really right now, but the price sure was . And I've been curious about mods for a while. Almost modded my old 1959, but it was in pretty good condition and I didn't want to "ruin" it. This thing I have now though.... is quite "ruined" already with its PPIMV and run-down aesthetic condition. I'm bringing it into the tech on Saturday - I knew it would at least need a general servicing, but it's not sounding quite right. I'm pretty sure one of, or some of the pre/power tubes are dying/dead, since the pre's are 40 years old and the power's are 20 years old.
Once it's back in tip top shape, I will run a TS9 through it just to see, but I'm pretty set on getting it modded in a few months. An extra gain stage will be added, along with a clean channel, and a few other goodies. I think I'm going to go with Dan Gower's KK++ mod. Once I get this back from the tech though I'll finally take some time to get good recordings of it and the Cameron.