Any kind of cab with any number of speakers could be 4, 8 or 16 ohms. I don't think you'll find many 16 ohm bass cabs these days. It's still possible.
If you try to run an 8 ohm cab with this thing, it won't be as loud as with a 4 ohm cab. The amp is rated at 4 ohms minimum, so you can't go below that or you'll risk damaging the amp.
There should be 2 speaker jacks wired in parallel. Plug in one 4 ohm cab, you're ok. Plug in one 8 ohm cab, you're not as loud but it won't hurt the amp. Still ok. Plug in two 8 ohm cabs, it will bring the total load down to 4 ohms - you're ok. Plug in two 4 ohm cabs, it brings the load down to 2 ohms. The amp could easily fry itself. It should suddenly be much louder than ever before that happens.
With the SVT Classic, I think there's a switch that lets it go down to 2 ohms. Very few amps will do this. Here's what happens:
Amps are dumb, sort of. A speaker is a resistor of sorts. Individual speakers could be 4, 8, 16 ohms or whatever - how many there are and how they're wired determines "the load". The speaker load actually determines how much power the amp will put out. If there's lots of resistance (ohms), the amp puts out less power. If there's not enough resistance, the amp will put out so much power that it melts itself. It becomes more like cutting a speaker cable in half, stripping the wires, twisting them together and plugging the cable into the amp - practically no resistance at all, but now you have a direct short. Some amps have switches to match specific loads - a lot of them don't. They all have a minimum ohms rating (maximum load) that shouldn't be taken any lower for the survival of the amp. Many heads should also not be turned on with no speakers connected.
There's a lot of 4 ohm cabs out there, probably the best match for this amp unless you intend to run 2 cabs.
The cab I have for my amp is a 2x10 rated at 8 ohms. The manufacturer also makes the exact same cabinet in a 4 ohm version. I wanted the option of running 2 cabs, but my amp is rated for 260 watts at 8 ohms, 400 at 4 ohms.
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