An 8 ohm cabinet won't kill it outright unless something else goes wrong, but it's not good for it either. The way an output transformer works, it "reflects" back the output device's operational voltage on its primary side (the "tube side") proportional to the resistance it "sees" on the secondary. (Since the actual impedance varies with frequency and a whole bunch of other factors, the "average" resistance rating is what the transformer is wound to accommodate and also how speakers are rated.) Bassmans are made for a 4 ohm output and when used with an 8 ohm cabinet, the operational safety factor of the amp is reduced by half on average and under certain conditions (port tuning, etc), even more because of the rise in the operation voltage of the output tubes every time the amp "sees" a resistance higher than 4 ohms.
Even with a 4 ohm speaker, there are all kinds of spikes and drops in the actual impedance curve at varying frequencies, (which vary with speaker, cabinet type, output voltage, and even room temperature) but by starting with an average mismatch of double the rating, the operational safety factor of the amp is halved. Bassmans run a pair of 6L6GC's at around 425 volts on the plates and a sustained (test tone into a dummy load at full output) 8 ohm load would result in them seeing 850 volts on the plates. If that plate voltage number goes too high for too long, two VERY bad things can happen: the transformer will start arcing internally and burn away the insulation between windings, "a slow death" that can go unnoticed until the amp starts having strange problems down the road or the tubes themselves can short across the plates and the amp pretty much blows up spectacularly and destroys the output tubes, transformer AND the speakers. Tubes are also made to accommodate operations variances, (they'd be useless for audio if they didn't) but their average ratings are also their guidelines too and the 6L6GC has a max voltage rating of 500 volts.
In real life, there are going to be short transient resistance peaks even with a correct load, which the tubes/transformer were designed for, but they're also going to be twice as bad with a load twice as "resistive" as a properly rated load. The Bassman is a pretty conservative circuit and it's not such a big deal, but in amps like an SVT where the amp is already skirting tube ratings under normal operation, too high of a speaker impedance is just asking for serious trouble. Your Bassman should be OK if you're not pushing it hard, but it's not a good idea for the long haul. The easiest solution is simply to add another 8 ohm cab. It's cheaper than replacing the speaker and you can change your tonal palette that way. And on another side note, the amp itself "sounds" different with too high of a load. The higher the mismatch, the brighter the amp sounds.
