Without having looked at the back of a Markbass tube 500, perhaps the 600w rating is the watts it will use when plugged in, and not the amount of watts that it will deliver to the speakers?
Ah cool, that's probably it!
Appreciate the replies folks by the way, usually I'm just a 'watcher' on the forums but thought I should take advantage of some knowledgeable people 👍
I thought there might have been an issue with the head somehow cos the replacement cab did the exact same thing after about 2 gigs lol when the distributed took the first cab away they said they didn't note any fault, but the guys in the music shop heard it same as in did, id have thought when a speaker went, that's it, but this seemed more intermittent. I ran the head with mids and highs boosted a bit but bass at 12oclock or cut at times, or I got a rubbish boomy tone.