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Generally, more interior volume is better. Mdf sounds darker, not as sturdy, ply is a bit brighter, durable, you can do things like adding some insulation material to simulate more interior pace, porting will allow speakers to move with less resistance from backpressure = more low end. If you want speaker breakup, try toget close to the amps max watts with the speakers total watts, if you want just what the amp does, exceed it by a bunch. This is all loose guidlines I have found mostly work for me.

If you want to buy a brand new cab I don't think Avatar can be beat for money/quality. Customer service is good, speaker selection is great. You'll go a little out of your budget if you want a 4x12, but that's due to shipping.

If new is not a concern then just scour Craigslist and used.guitarcenter.com to find a good deal on a quality build. Mesa, Marshall, Orange and the ilk should all be good, you'll just have to see if you're happy with those speakers.

Like most things your mileage will very with anything you pick without playing it first. I feel like if you were moderately happy with 70/80's you'll probably be happy with anything that's an upgrade. I think the 70/80's are the "ideal" speaker for modeling amps because they don't color very much but maybe I'm confusing that with something else. I haven't played them personally.

If you are lucky, your amp will play well with a Marshall 1960 cab loaded with the standard issue Celestion G12T75 speaks. Lots of those out there to grab cheap off of Craigslist if you need that much cab. Some small scale amp builders don't fuss with doing their own cabs and just voice their amps to those. Kind of a no brainer strategy for the builders. And the cabs can sound really good with the right amp.

Don't bother with a 2x12. If thats what's appropriate volume wise, then trade in for a combo.