| If the cab is 8 ohm I would've recommended the WT800.
But 4 ohm, you simply do not need the extra muscle of the bigger amp.
Basically, you are good with either head. The bigger head, would just be a nice-to-have, and perhaps gives you more flexibility for future changes / upgrade in the rig. If you are worrying about over-powering the cab with the WT880, then just be careful with the volume knob.
I don't have experience with the exact same config, but I tried both head with a couple of different 610s (Eden, SWR, Bergantino...) and they are all slamming, even with "only" 550 watt from the WT550.
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