Hi Roland and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, wire two (2) speakers in series which would result to 16Ω and connect to 1 of the mono jack. Do the same for the other 2 to the other jack.
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If only 1 jack (hence only 1 speaker pair) is connected to the 333XL, then set its IMPEDANCE selector switch to 16Ω.
On the other hand if the 2 jacks from the cab are connected to the 2 jacks of the Bugera, then set the IMPEDANCE switch to 8Ω.