Disclaimer: Not that familiar with any sort of metal.
Before throwing money at the problem with more gear, try the following...
All of these steps can be done with little to zero money added. So even if I post a video that has a product for sale, the intent is to do this without spending any more money.
Step 1: Re-EQ your guitars
I would start-over with twisting the knobs of your guitars. In other words, re-EQ. I know most simply dime both the volume and tone controls but maybe try having both the volume and the tone knobs at the top of the "sweet spot" rather than just dimed. That gives you some adjustment room.
Step 2: EQ your amp
I believe the key to high-gain is roll-off (reduce) the bass in the pre-amp section and add it back after the pre-amp and before the power-amp. This is what the "resonance" knob is supposed to do. So drop your bass knob to 0-4 and increase the resonance knob correspondingly.
Generally, people who play at low levels while practicing turn the treble way way up because it sounds "clearer" at that lower volume. Then at high volume it turns treble-y. I would get that clarity from turning down the gain a-bit and bumping the master volume a-bit.
The 2 standard approaches to EQ-ing your sound seem-to-be...
1. Find the "sweet spot" of dial sensitivity and adjust from there
2. Start with everything maxed and turn down what you don't like
I would do a hybrid and start with your EQ at the sweet spots for each and then subtract what you do not like.
https://www.youtube.com/watch?v=k1dRBLZu1Og
I would start this process with a tone that is as clean as possible in each channel (drop the volume/gain to 0, turn up the master volume, and then add just enough gain to hear it well.
After you EQ each channel with a clean tone, you and dirty-up your dirty channels by doing the reverse. Add the level of gain you want (top of sweet spot) and then adjust the master as needed. Make slight adjustments to EQ.
Step 3: Adjust your pick-up height.
The first video really demonstrates the warbling you get with pickups that are too close. I know he is not your genre but the rumor is that SRV dcrewed his pickups to be as far away as possoble so he you almost dime his amp.
https://www.youtube.com/watch?v=nqE1xMUvVq4
https://www.youtube.com/watch?v=2biWMx6q8U8
Step 4: Block the beam with some DIY
Sound is very directional especially at higher frequencies. That is why your home theater systems has only 1 subwoofer but 4 high-frequency speakers to do the surround. There is a phenomenom called "beaming." Here is video showing beaming and 2 videos on cheap, DIY fixes for beaming.
https://www.youtube.com/watch?v=u_BaO7Mg3oQ
https://www.youtube.com/watch?v=Ofy8Jz-HxBo
https://www.youtube.com/watch?v=bmECNGU-4HM
Step 3: Reduce your amp (or its power)
The Marshall DSL40c is a huge amp. Probably a little too large. Run it on half-power so you can push the master volume past 1.75.
Hope this helps.
