I have a TecAmp Puma 1x10 combo, which you list as one of your alternative choices. The 1x10 appears to be discontinued, but the 1x12 still exists, thoman have for more or less your budget depending on the exchange rate.
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1. Does the small cab size make it sound boxy and thin?
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No, not at all. You can EQ loads of bass back into it.
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2. Is it hand portable, in your opinion?
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Yes, but I wouldn't want to carry it much more than a mile by hand. If you could put it into a rucsack or a set of straps and carry it on your back it would be fine. I've transported mine a short distance on the carrier of my Dahon folding bike, to prove it could be done more than anything.
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3. Though the official website strictly prohibits it, is it possible to replace the amplifier with a different model?
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No. You'd need to take it to bits.
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4. Is 12" really enough speaker coverage to play with a drummer?
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The 1x10 copes well enough, so the 12 will be fine.