I've never been able to get anything really chunky out of it, it has that 5150 flavour as such, but has really pronounced high mids, that are hard to dial out. I can virtually nail the tone on Hate Crew Deathroll using any guitar with passive p/up's and a Boss SD-1 (in place of the JE-1000) into the GP-1000. Sadly active EMG's don't seem to work well with this preamp, it always comes out sounding weird, and nasty somehow.
As far as having the mids cranked as Ben said, that doesn't really seem to make a difference on my GP-1000. On mine, when the Mid Shift hits 3 or above, pretty much the only EQ knob that does anything is the Treble knob, turn it down and you lose gain and volume, and the Bass and Mid knobs only make subtle differences from being on zero, to being maxed. But having the Mid Shift below 3 makes the preamp sound really thin, and it loses any balls it had in the first place. It's really fucked up how the EQ section on it works.
Basically Alexi's rig is as you posted initially, the only thing that's really changed over the years has been the poweramp. He did also use a different preamp from time to time as well, but most of the time it's been the GP-1000. With decent passive p/up's you don't really need an extra gain boost, the GP-1000 unlike most people think, actually has a decent amount of gain by itself. I prefer the tone without the boost to be honest as it's thicker, and less mid-orientated.
Overall a lot of Bodom's tone comes from how Alexi and Roope play. I think with a JCM800 and a Boss SD-1 you could pretty much nail the tones as well. Or even a 5150, or the like. I've seen Alexi playing through Marshall heads in videos in the past, and it doesn't sound far off the album tones. Also the Marshall or 5150 route is more versatile as well, the Lee Jackson will pretty much do the Bodom tone, and not a lot else (although using GH's Fredman Impulses, I've been getting it to sound kinda thick, and not too bad). Also the clean channel is really shit as well, even worse than the 5150 clean channel IMHO.
Ok, Bodom/GP-1000 rant over, anything else you wanna know, just ask
Edit: Is that enough shit for you Bob and Kev?