A couple of clarifications on this discussion...
If you wire four 16-ohm speakers in parallel, you'll have a 4 ohm total impedance. This is the same as half of an SVT cabinet and is a good way to go.
The second jack on most speaker cabs is usually wired in parallel so you can add another extension cabinet. But this will lower the overall impedance seen by the amplifier to z1+z2/2, where z1=1st cabinet's impedance, z2=second cab's impedance. If the cabinets have different impedance, they will also draw different amounts of power when wired this way, so they may have different loudness as well.
The Marshalls with a splitting switch let you split the cabinet into two 2x12 channels in the same box. If you run a stereo effects rig, then you only have to carry one cabinet which is nice, although there's not much separation for things like stereo flanging.
Solid state amps can drive variable impedances and produce different amounts of power depending on the impedance it sees. The math is fairly simple
Output Power(in watts) = Volts (AC) * Current
For solid state amps, the voltage largely stays the same independently of impedance. The current varies in inverse proportion to the impedance. So if you double the speaker impedance from 4 ohms to 8 ohms, the current will be halved and you'll get half the number of watts out. Similarly, if you halve the impedance from 8 ohms to 4 ohms you'll double the current and get twice the power out. If you connected two of those 4 ohm cabinets to this same amp in parallel using those cabinet jacks, you'll halve it again to 2 ohms and double the power output again. Yes, the same amp that produces 100 watts at 8 ohms will produce 400 watts at 2 ohms.
But for this to really work, the amplifier has to be designed for this much power. When the power transistors are cranking it at 2 ohms, a lot of current is flowing and they will get very hot, so there needs to be sufficient heatsinks and airflow otherwise the transistors will literally burn up. This is often the difference between true pro gear and weekend warrior stuff and accounts for some of the big difference in price.
With most solid state gear, 4 ohms is not a problem and 2 ohms will be dicey. 8 or 16 ohms is no problem, and your amp will run cooler, but it will be less loud.
Now, remember, your ears have logarithmic response, so 2x the power is only a 3dB increase in sound level which is only a little bit louder. For something to sound twice as loud, it will need a +10dB increase which will require 10x the power. So don't fry your amp for a small increase!
Tube amps require a matching transformer between the output tubes and the speakers. With tube amps you always want an exact match of speaker impedance to the appropriate output. A Marshall has an impedance selector switch which reconfigures the output transformer. If you set the switch wrong, you can easily cook the output transformer if you play loud. You probably never will want to connect a tube amp to a 2 ohm speaker.
David Fung
