#1
I used the search function and didn't find anything else recommending pickups for this guitar. At most the other threads I've seen suggest that I get a high output passive for the bridge and a medium output passive for the neck, which is what I was going for anyway because I don't want to use actives.

At home I run this guitar through a Marshall MG50 and a BOSS ME-50, it still has the stock pickups in a Humbucker-Single-Humbucker setup and I play in standard tuning most of the time.
I also play a bunch of different amps at college, including a Line 6 Flexitone, a Marshall MG100, a Fender and some Peavey amps.
I play everything from Jazz to Metal so I'm looking for something that can give me nice tones for any style, which I'm sure you'll be able to help me with

Clean: I can get a nice clean sound when I use any of the 3 middle pickup settings on my 5-way converter, but these always come out quieter than the neck or bridge. The neck pickup can sound a bit muddy at times if I'm playing open chords but the bridge tends to be a bit too bright, leading to me using the middle pickup.

Distorted: The stock pickups sound pretty muddy if I put on too much bass and I can't get a nice lead tone out of the bridge because it gets too trebley, so I have to switch to the neck which gives me a nice solo tone for highs, but horrible and muddy for lows.

I was thinking of getting a DiMarzio D-Sonic for the bridge because I've heard good things about it in reviews and forums, but if you'd be able to recommend me anything else I'm open to suggestion because I don't really know much about pickups.
When I get some more money I'm looking to get an Air Norton or a PAF Joe in the neck, but right now I'm looking more towards getting a nice sound from the bridge.

I hope I've included all the information you'll need
#2
Its honestly your amp, the Marshall MG series is pretty awful for lead tones, got a decent rhythm tone though.

But if you want pickups, Bareknuckle get raved about more often, then there is Seymour Duncans.
Get a tb4 for thebridge and a 59 for the neck. This is what Jacksons have, and I used them in a shop, I liked them, but not the rest of the guitar. For clean and metal distortion, it is dam good. Not sure about your jazz....
#3
Yeah, I know about the amp. I'd get a new one but it's not really in my budget right now, forgot to include that. Looking for something below £150. I don't really use the amp's distortion anyway, it comes more from my ME-50 which I've also heard isn't amazing but messing around with it I've found a distortion setting that I like. More of a problem is the Fender amp I have to use at college sometimes.. I'm not sure if its broken but everything sounds terrible and the EQ hardly seems to make a difference, it just seems to have too much treble and yet its still very muddy.