Wrong pedal
I agree
A tube screamer or any overdrive pedal will add another color layer to your distortion cake (i.e. your marshal's lead channel), but it won't make it sound heavier like a 'Killswitch-genre' tone! The heavier stuff is best achieved with less gain sometimes, believe it or not, with more low-mids and getting the right presence. What kinda guitar are you using? A les paul should match pretty good with your current amp to give you a heavier tone, but keep in mind... if you want that tone? think VHT, Mesa Recto or something similiar in that mind. Also, more of the heavier chug chug stuff is sometimes not even achieved with a tube amp, Wayne Static from Static-X uses those solid state marshals. Dimebag used solid state Randalls for the most part.
But all in all, not to get carried away, (you dont need me to tell you that your marshal is not a great and capable amp), but keep in mind other variables such as guitar choice, pickups, fretboard wood, string choice, lower alternate tunings?, pick gauge, all play into this...
I went on and on again, damn...
But yea, it's tuff to pick a stomp box that will get you really heavy, yet still sound natural and sweet like a fully pushed tube amp. The best tones do come from an amp used to its full potential, even those really heavy ass sounds. Keep in mind that a lot of 'heavier' bands these days like Mudvayne still clean up too so versitility is still necessary. Oh yea, what kind of speakers are in your amp? Is it a 4x12 setup? How's the condition/type of your tubes? 6Ls and 12axs sound kinda different...
Anyways, i can't stop...
As far as pedals go...
*Try
a Tech 21 Sans Amp (which would probably work best if you layer it over your already driven and distorted amp, when compared to any other stomp boxes)
*Boss Metal Zone (best in front of your clean channel, wud sound like ass over a fully distorted channel, maybe better if you almost completely kill the gain on your amps lead channel though, but this baby has chug chug character no doubt even on a little practice amp)
*
Voodoo Lab Super Fuzz (although this thing is a fuzzy beast, it still has some smooth low end)
*Electro harmonix Big Muff- (still along the lines of the Voodoo labs but Cobains heavier stuff was on this -i know Nirvana guit tone not that close to Killswitch)...
Which leads me to...
You can go to every music store and try all this out... and it would benefit you to try it out on your current setup/ signal chain... but man, that is some serious searching as there are many pedals out there. My humble advice would be to make sure everything except a pedal is up to par with your tone (as in guitar choice, strings, speakers, pickups, etc as mentioned b4). Who knows, like my once lonely toneless road, you might want to ditch the stomp box distortion pedal as your primarly heavy sound generator and go with a great amp's channel. Your marshal isn't a VHT, but with the right conditions it can still drive. But yea, the Boss Metal Zone is probably your best bet if you do go the pedal route and while you are at it... your heavy tone will benefit from a noise gate like the one Boss has to keep it souding tight...
Sorry for the ass long response... PM if you need any thing
Thanks