The Boss MT-2 Metal Zone pedal is a pedal that finds itself frequently maligned. That said, it's been the staple of many classic bands' most significant records, from Cannibal Corpse (who used it as a primary distortion pedal on their early albums and even as a boost pedal for their Dual Rectifiers later on), Deicide and At the Gates. I also discovered it was part of the secret ingredients to other known bands, praised for having a unique sound.
The truth is, it's not my favorite pedal, but it's not nearly as horrible as many make it out to be; you need to know how to use it. The tone controls, in particular, are extremely sensitive, and tiny changes can make or break the sound. A lesser-known fact is that it sounds much better plugged into the amp's FX loop than it does into the pre-amp section too. I'm not sure if it was initially designed to be used as a pre-amp or if it's just a happy coincidence. Either way, the FX loop makes a difference.
However, a straightforward "solderless" mod that I discovered online was a giant leap in sound quality for the pedal. My electronics skills are basic, so I feel confident that anyone with a soldering iron who can repair/build their guitar cables can do this (though I won't be held accountable to anyone wrecking their pedal). There are different versions of this mod, but they all revolve around removing various capacitors from the pedals board. The version I settled on was removing the C24 and C35 capacitors (they're labeled on the board). I also tried a version where I removed C25 instead, but I preferred it with C24 removed.
To get a little more technical, removing the C24/25 capacitors means that the frequency range of the distortion is evenly distorted. In contrast, when they are left in, the frequencies roughly below 1kHz are less distorted than the higher frequencies. Removing it makes the sound more rounded and less harsh - a common complaint leveled against the pedal.
Removing the C35 cap results in a mid-range frequency boost before the distortion circuit being disabled. This can result in a more nasal sound coming from the pedal, and once again, with it removed, the pedal sounds more balanced.
I made this video to highlight the differences between each stage of the mods, showing the pedal in the amplifier's pre-amp and the FX loop. Please remember that the different settings aren't initially set to sound "good" but are to highlight the differences. I did include a take at the end with my preferred settings where it sounded good to my ears. However, most of the video is intended to demonstrate the changes as accurately as possible.