The more I walk the Diezel path, the more I realize that a lot depends on your playing and what you plug in these amps.
I mean, they're so sensitive and responsive that you can achieve all the tones you're looking for without messing with a soldering iron.
Imho you could start by lowering the gain and see what happens: when I first got mine I felt like sometimes I needed more gain (especially on CH3) but after a little while I realized that I had to adjust my playing style to really make the most out of its tightness. There was already way more gain that I actually needed, I just had to learn how to manage it. To me the gain structure on the VH4 is so unique that this "calibration" step is really the first to take, especially before making mods.
If this doesn't work, then you can start looking at your chain: type of guitar, pickups, cables, effects or every other device you put between your axe and the amp (bad cables, bad buffered stompboxes and stuff like that tends to suck a lot of high frequencies).
Also the cab makes a whole lot difference. Sometimes you lose your sleep behind a cap, a resistor and this kinda stuff, then you try another cab/speaker and you instantly get a completely different tone.
At the end of this journey, if you really can't achieve what you want, you can start looking at the amp (preamp/power tubes first, then mods), but I wouldn't go that route before trying all the other steps first.
This is just my opinion based on the direct experience with my VH4, hope it helps!
Good luck!
Paul