Well, Gothenburg = Peavey 5150. If you like that tone, that's the fastest (and conveniently, cheapest) way to get there.
I play mostly seven string guitar (in B standard, but I've farted around as far down as low E, an octave below standard tuning, before), have for the last six or seven years, and moderate over at
www.sevenstring.org where we have several thousand members who also play extended range guitars. As such, I've got a lot of personal experience and I've talked to a ton of people who've also had a lot of experience, so I sort of feel like I can answer this with a good degree of confidence for you.
Basically, the difference between E and B isn't THAT great, so most amps you can get a metal tone out of for E standard will cut it in B. I go back and forth on Mesa's Mark-IV, which B is right about the limit I'd take it to, but then again I've never gotten to reallly open one up before so maybe the poweramp is what you need to give that thing adequate low end definition.
However, while anything within reason can handle low B (within reason in this case means adequate headroom and at least 12" speakers - I wouldn't recommend trying to gig with 8" and 15 solid state watts), you really need to spend some time dialing your amp in right. The long and short of it:
GAIN - with low tuned guitars you really need to be sensible here. It's true of anything of course, but especially so wiith super-low notes; if you jack the gain all the way up, you're going to lose a lot of the "impact" of your tone as the initial attack gets compressed to hell and back, and really the attack is what lends a ton of definition to your tone. So, when in doubt try less.
BASS - I know, this one's obvious, but if you;re trying to reproduce low frequencies, you need a lot of low end, right? Wrong, actually - because you're sending so much low energy into the amp, you need to be careful with the low end to keep your tone from farting out.
To put things into perspective, when I'm dialing up a heavy tone, I generally don't have the gain over 5-6 and bass about 2-3 on the Modern mode of my Mesa Rectoverb. Granted the Recto series isn't the greatest example since the throw of the bass knob is designed as a compromise for alll three distortion modes and as such has WAY too much low end for the (already bassy) Modern, but the 10-0-10 mid scoop a lot of metal guys start with will rob your tone of all definition.
Other things to try:
Use an OD - this is the Andy Sneap school of tone, a Rectifier (or some such amp, a 5150 would work here too) with a Tube Screamer out front with the gain at or near zero, and the level about unity so it doesn't actually really boost the signal. The idea is not to add additional gain (though you are, a little) but rather to use the pedal as sort of a pre-EQ to tighten up the low end a little (as part of the TS magic is the way it lops off a bit of the low end). If you're curious, I've been using a
Bodenhamer Electronics modded TS9 - he mods tube screamers especially for this purpose, but to be fair mine also absolutely slays overdriving a clean channel with a strat for SRV style tone. I've been meaning to grab one of his Bloody Murder OD's as well, simply because it'd crack me up to show up at a blues jam with something called a "Bloody Murder."
Try Lighter Strings - This one's, well... I string heavy (10-68 on a 7) simply because I hit the strings pretty hard and I need the tuning stability heavier strings offer to stay intonated when I pick, but even I'll admit that lighter strings are brighter and less compressed than heavier strings, and if you're trying to preserve attack through a distorted tone, this is a good way to help that. There's this prevailing "heavy strings = more tone" manliness thing, and again this is coming from a guy who uses telephone cables, but it's not "better," just different. For some applications, lighter strings can rule. If you don't pick so hard that this is a major issue, then you may want to try going down a string guage and see if that improves your clarity.
Remember, though - for low tuning, always strive for clarity, attack, and definition to preserve your guitar sound in a mix.