Hi Jay, and welcome to WholeNote. The Marshall 2203 is one hell of an amp, talk about power, those things are monsters. I know, I own two of them. The first thing you might want to try is swapping the Crate 4X10 cabinet with a good quality 4X12 cabinet, preferably loaded with Celestion speakers. A 4X12 Celestion cabinet will drastically improve the sound of the 100W Marshall. A new set of good quality tubes always helps. One of my 2203s has Mesa tubes, the other has Groove Tubes. The Groove Tubes distort more, but the Mesa tubes are "tighter" and have more punch (Just my opinion)
Don't be too hasty when deciding to "mod" your Marshall, some Marshalls didn't take kindly to being modified and as a result their sound became very "tinny" and "brittle".
The only mod I ever knew of that worked (sort of) was called a Variac, basically it is a high voltage, adjustable transformer that is wired into the amps power stage. The problem with that is, output tubes burn up on a daily basis, the amp is no longer reliable, and the gain is coming from the power tubes, not the preamp (not good).
I use a graphic equalizer (pedal) to preamp my 2203s, just set the EQ in a "reversed scoop" so the mids are all the way up, this way the EQ acts as a preamp, and the 2203 will have alot more gain at a lower volume. Some overdrive pedals work very well with the Marshall too, tho I've never used them, my EQ always worked well for me.
Good luck.

Before taking an iron to such a wonderful beast, I suggest a) getting a better cab with 4 12s like the other guy suggested, preferrably celestions, but you could buy a cheap cab and upgrade those later, or, if there's room for expansion in the cab you have, you could fool around with it some. If so, you could maybe buy two speakers at a time, which would certainly be easier on your wallet than buying a brand new celestion-loaded cab b) experiment with different power tubes, like KT66s or 88s (i can never remember which one works for that, or maybe its both). Also, feel free to try different preamp tubes, get one with a lower gain rating than the ECC83s that are in it (I'm guessing anyway), and switch it around in different sockets, this, I've heard, can durasticaly decreae the volume without harming anything or changing the tone that much (I read that somewhere, I'm thinking its the first socket). Also, if you havn't done it in a while, and especially when/if you get new/different power tubes, bias the amp (!) that can make quite a difference in tone and make your tubes last a lot longer, win win situation. Have fun, but I don't suggest having an iron anywhere near something that awesomely perfect, not to mention finicky, too easy to screw something up. I hate to say this, and I'll probably get a lot of lip about this, but if it's anything electronic (guitar amp or otherwise...ever heard of lucas electronics? wow) made in Britain, if it ain't broke don't fix it.

The substitute for EL34's Are KT66's.
The Chinese KT66's need to be biased hot in order to sound good, but the Russian one use a bias vale very close to the EL-34's so you proabably won't have to rebias.

If you are not used to working on tube amps I stronly suggest that you either go with tubes that are biased the same or let an experienced tech do it. These is up to 500 VDC present at the tubes. That's enough to kill you!!!!

Wow. WOW!! Thank you all for responding!! =D I was already thinking of getting a Marshall 1960A cab, and consigning the Crate cab to the local music store.
My setting on the JCM are:
Presence: 9
Bass: 4
Middle: 0 (I hate hate HATE mids! LOL )
Treble: 5
Master: 1-2
Pre Amp: 10
Oh, and usually play a Dean Icon or Hamer.
I run a BOSS ME-50 pedal and a DOD EQ...I keep it low because of the neighbors and because I run from the cab a cord straight into a 4-track, though I have been considering miking the cab.
Right now for tubes it has 6550(?) Groove tubes and the obligatory 12AX7's.
Well, Im going to take your suggestions and leave well enough alone...the harmonics you get from this amp are amazing..no transistor could ever make that kind of sound..and yeah, Im familiar with Lucas...I used to work on Jaguars, and now I run a car parts store; my suggestion with pre 1994 British cars: Stay away....stay away...lol

Jay

12AX7s, 6550s ... Groove Tubes, bro, you are well on your way to an awesome f*cking tone!! I can see by your settings that you like the same tone as I do. All I can say is "Mike the cab!!!!!" You will love the tones you can achive. Try placing the mike in different places, you will be AMAZED, it's actually better than EQing.

if it has 6550's in it, ditch em. go with EL34 for the classic marshall tone. you will need to rebias the amp though.
get rid of the pedals and run it straight out..

if that doesnt work, there are plenty of mods that can be done to it. what do you want it to sound like?

germ

OK...you should go from cheapest to ost costly on upgrades, since you KNOW you have a great amp...if you can do the work yourself, tunes and speakers are the cheapest upgrade. Next, based on your complaints, maybe an EQ pedal is your next choice?

I have been myself considering a Powerbreak, since those SOBs who make amps give all the optiions to the high-output crowd...