I believe Zakk's signature speaker is just a reissue of the 300w EVM12L Pro-Line.
Zakk's main Marshalls are all stock 2203s, except with 6550s. I believe the switch to 6550s and EV speakers came around the same time in the late 90s.
His signature chain is weird. Uber tone-sucky Monster cables into tone-sucky wah and rotovibe. His OD has all knobs on ten. His amps are generally dialed with bass and gain maxed, presence and treble around 6, middle at 5 and master at 3 or 4. I find his amp settings to be quite muddy without the OD, but with the OD at full blast it balances out. He says he dials in his gear by putting everything at max and taking away anything that's too much.