Boss MD-2 or MT-2?

No idea about the MD-2. Spankypants did a bass demo of the MT-2 a while back, it seemed to hold the bottom end pretty well there.

Ahh, here's the demo:

http://www.talkbass.com/forum/showthread.php?t=405449

 

I use a Metalzone on my bass.

It's not very suitable if you play it fast, especially if you move from string to string.
(So no Cliff Burton like playing possible.)

But it sounds very nice with the distortion turned halfway and a bass equalizer in front of it.
The effect is kinda like a fuzzy midrange, metallic distortion in the highrange, and sharp overdrive in the lowrange.

I use a Ibanez Phat Hed for fully metal distortion, though that pedal I set in overdrive mode. Go figure.
The gain on the amp at ten helps.

So one low gain, one high gain philosophy.

I personally had no bad experience with it.

Other things I can do with it:

- distortion a third up, boost lows and highs a bit (to your liking according to your set up), play with a pick on a fretted bass (I play fretless mostly), a very cold ambient spine shivering tone for intros and interludes.

- even with distorion around 8 (scale 1-10) it brings somwhat emphasis on your playing technique. It's like the effects of hammer-ons and pull-offs are accentuated. Even tapping sounds extreme.


You can't go wrong with, but it all depends on your style.
What do you want to use it for stylewise?

I can't say anything about the MegaDistortion.