Right, first things first, I only have £200. More can be acquired, but provisionally for the sake of the thread, £200.

Currently my rig is as follows:

Vintage V100WR>>delay pedal>>Randall RG50TC. I have a GE-7 which I don't use, so that's just sat in its box.

The guitar has mini humbuckers, and I have a feeling this is part of the cause of my problem. I like the guitar in all other aspects, it's just the mini humbuckers are quite muddy, and empty. The only other complaint about the guitar is the neck isn't quite right for my hands, I don't know why, it just isn't.

The amp is far too metal voiced, since I'm chasing a Marshall-esque sound. It also feedbacks too loudly when cranked, and when cranked, the low-end causes it to rattle. I've been looking at the dsl401, and there's a JCM2000 combo (forget which model) going on ebay in my local town for £350 BIN.

Now, where to begin? The amp might be O.K. with a better guitar, and likewise the guitar might be O.K. with a different amp..
You could replace the pickups and buy a noise gate to get rid of the feedback on the amp and maybe just try eq'ing a little differently. If not, I think a new amp would be your best option since your amp has a lot to do with your tone. Once you have the amp you can always go ahead and just replace the pickups if you really want to but at least you'll already have an amp with the sound you're looking for.
Quote by csn00b
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maybe you should have new pickups installed. it might cost around 200 because if your guitar has mini humbuckers they might have to make room for full humbuckers. look into burstbuckers or maybe seymore duncans.

Epi LP Custom EMGs
ESP H-1000
Schecter Elite 7
Jackson Dominion
Peavey Valveking Head
Jet City JCA24s
Blackstar HT20 & HTV212
MXR Pedals
Schecter Stiletto Custom
Schecter Stiletto Studio
Fender Vintage Modified
Ampeg B2RE
Fender 4x10 Cab
I'd say go for a new, more british voiced amp for sure. Start with that and of you find you still dont like your guitar, get a new one. The DSL is decent, but if you have the cash you could look into something like Ceriatone