I think PBG is overstating the risks of running an SVT into an 8 ohm load. Here's why:
Those writeups do a good job of explaining why tube amps must never be run with no speaker attached (theoretically infinite ohms) but are relatively safe into a short circuit(theoretically 0 ohms), while transistor amps are just the opposite.
But that's not the situation in this case. Running an amp into an 8 ohm cab is very, very different from running one with no speaker attached, for the simple reason that 8 ohms is much closer to 4 ohms than it is to infinity (or millions of ohms, which is the situation in reality). Assuming good tubes and a good output transformer, an SVT is not going to die from running into an 8 ohm cab. If it does, either the amp was was dying anyway (probably from having been run previously without a load attached), or the speaker simply blew and failed open, which can happen with any speaker regardless of its impedance rating.
As PBG correctly described, higher impedance on the secondary or output side of the transformer causes higher voltages to develop on the primary or tube side. This is the mechanism that kills transformers when there's no speaker attached (which equals very high impedance); the voltage on the primary can get so high that it destroys the output tubes, punches through the wiring insulation in the transformer, or both. In the case of an open output (no speaker), the voltage spike can get up into the range of thousands of volts, and few audio tubes or transformers can withstand that. But using 8 ohms instead of 4 does not produce that kind of voltage rise, and only defective or marginally-designed equipment (and the SVT does NOT fall into that category) will die from an output impedance mismatch of 2:1.
I know PBG has a good rep here, and you may not believe me, and that's OK.
You do have another option, and in fact it's the best one. It's pretty easy to rewire a 4-driver 8-ohm cab so that it beomes a 2-ohm cab which the SVT can drive. If you're not up to that yourself, any good electronics tech can do it, and they can even add a switch so you can run the cab either way. Issues of amp stability aside, you'll get the best sound and best power from doing that.
EDIT: If the SVT has an extension speaker jack rather than a switch (I know the oldest ones did), and you don't want to modify the SVT, you can still rewire the cab, but use two input jacks instead of one. The cab effectively becomes two separate 4-ohm cabs in one box. Then run a cable from each output on the amp to an input on the cab (two cables, in other words). You can still have a switch on the cab to allow 8-ohm operation as well.
Hope this helps.
Reid
