Oh interesting, I didn't know they made a 15-watt version of that amp. Nice!
You should be able to get SOMETHING out of that - whether or not it's exactly the right amp for you is another question (and one I recommend answering before you buy something in the future), but you should be able to at least get a decent hard rock or metal sound out of it (I always thought that amp was a little better for dark hard rock sounds, personally).
I don't remember exactly what the rhythm settings I used on its' big brother, the TSL, but I believe it was somewhere in the ballpark of gain 6, bass 4, mids 6, treble 4. Don't use the Deep mode, it just adds mud, and leave the Tone Shift button in "normal" mode rather than scooped mode. Also, try running it as loud as you can get away with in 7.5 watt mode - volume really helps marshalls. Also, the TSL seemed to have less gain than the DSLs I've played, so you might want to try the gain a little lower, maybe 5 or 4. Too much preamp gain on a Marshall will give you instant fizz and mud, which sounds like your problem here.
A OD as a slight boost to the front end could help, but if you're not pretty close to the sound you want already it's not going to be a silver bullet.