You are talking about the 1000S you mentioned in your other posts, correct? If so, you have 120 watts, not 280 watts, and should have no problems with a 240 watt rated cab. The tone you will get should be all amplifier without any speaker breakup of any consequence.
Hopefully, you won’t need a new output transformer. There may be others, but the only accurate replacements I have found are from Mercury Magnetics, the Axiom Tone Clone Series #SUNN2000-O. They are listed as for the 2000S, but your amp is identical in the output stage. You can find them at;
www.mercurymagnetics.comThey are $400 plus shipping unless you order multiple units.
Without good transformers, your $300 amp is almost worthless. Other than MAYBE the tubes (depending on make, vintage and condition), the other components are a dime a dozen.
If it’s in good, clean, fully functioning condition you got a steal at $300. You could double your money on ebay fairly easily.
By the way, in the other post about the 1000S prototype those guys are talking about a smaller 24" wide 100S chassis, factory modified to deliver similar output to your 30" wide production model. They truly are rare and valuable. I want one.