just some personal input. Definitley sell the metal zone and get a boss gt-8 or gt-10...but be prepared for some countless hours of tweaking or just don't get it.
I have a boss gt-10...just picked it up last week...and so far I really like it. I previously owned the gt-8 a few years back and didn't really like it, I didn't take the time to get in to it, I wasn't as patient. But after going from that to multiple noisy single stomp boxes, then to an overpriced but beautiful sounding TC G-System with some OD stomp boxes...I found myself researching a bunch and ending up back at the BOSS route with the new GT-10. I have to say I am very happy, I got most of my money back after selling that G-system and the effects in the GT-10 are pretty close to the same quality...not quite...but close. But you get so much more.
for opeth tone, I definitely find it best to use a BOSS GT-(insert model here). When you first go through the presets you are going to think "why the fuck did I waste my money on this"...but really you have to spend time at it because all the good tones are hidden in tweaking.
it is best to start from scratch (ie. turn everything off and start from the pre-amp). for me, i use the 4 cable method, dual mono 5150 drive preamp, 1 of them is mid scooped, 1 is mid boosted a bit, very low gain (around 40/100), and a resonator to fatten it up a bit. I am still playing with the eq and speakers to get the exact sound I want but I have to say...for a digital distortion this is probably the best I have ever heard so far
the great thing about these units is you can turn the pre-amp off and use your amps pre-amp if you like it, and then just colour your existant tone with effects/od
they are extremely versatile and very very convenient...you just have to spend time at it
edit: it also helps to have a cu24 with a SD full shred in the BRIDGE, not the neck, the prs vintage is much nicer. and I wouldn't necessarily say the full shred is weak...its just not harsh and not mid scooped like the dimebucker or something. its a warm fat and full sounding pickup but not tooooo hot. though...tone is so subjective isn't it haha