as the title says, what is the heaviest one of these amps can go?

has anyone pushed them into metal territory?

also, in theory, can you basically use an over drive pedal to push any valve amp into high gain?

They aren't metal amps by any stretch of the imagination, I personally wouldn't call any of the sounds they make "heavy"...heavy sounds to me says lots of low end, distortion and compression. The Vox is a bright amp and the sounds pretty "open", even with extra gain it's not going to sound "metal".
Actually called Mark!

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Yeah, the voicing of the amp matters as much as the gain it has. You can add gain, but you can't change voicing, not without a ****load of effects, and then it just sounds too synthetic.
I own an AC30 and have had it dimed many times, with a Maxon 808 pushing it....

It gets REALLY nice and crunchy.... sweet sweet overdriven tube sounds... but not what I would interpret as "metal" tones.

I would say that an OD can help move closer to "metal" tones (I really think that the definition of "metal" tones really spans a wide variety of interpretation, may be you can clarify which kind you mean)... but the amp is going to be a bit more crucial in finding teh br3wtaHlz you are looking for.

i.e. I also own a Fender Twin and a Valve JR.... both tube amps that sound great... but even when dimed and with the OD I would not say they have "metal" tones at all....

Now I have heard (I have not played through one) that a Peavy Classic 30 has AC30 like tones and can be driven into the Metal Territory... may be check out one of them!
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The other guitarist in my band uses an AC-30, it has a great clean sound, but obviously you have to crank it quite loud to get it dirty. He does use it for heavy sounds but only in conjunction with other amps. It's not a typical "metal" amp but it adds some nice mid/top end to the overall sound.