So, What happens if you run a 4 ohm load with the speaker cable plugged into the 8 ohm output jack? I hooked up my 400+ that way last night at the gig as I'm used to running just one 8 ohm cab. I got a second cab but spaced out plugging into the 4 ohm jack.
You can better short-circuit a tube amps outputs (impedance = 0 ohm) than to run it without a load (impedance = infinite). Best thing is to match amplifier and cabs obviously, but between the two evils you did the least harmful for your amp, so don't worry.
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