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Gomez wrote:I noticed there's some buzzing in the cab when playing low notes... could it be the speakers or is it more likely to be the actual cabinet?

Could be a loose back panel vibrating against the center post. Try pressing on it with your knee (or get a volunteer) while you play the low notes to check.

Gomez wrote: I think it's in series.

Can't quite make it out on the picture .

Again, if the positives and negatives of both speakers are hooked together, it's parallel.

Dave