DSF

This is always the first question to check ink dates. Personally I don't believe it would make that much difference if they're not long over. Yes, it will make a difference but not this much. Did you get them to check all settings?

One of my printers had inks over a year out of date, the colours didn't change but tones did, almost like they were weaker.

Maybe this is the time to look at a different inkset? Give SG the chance to dial in and check everything first and then make your decision.

Also, you mentioned your nozzle check was ok? If colours were ok in this they can't change as drastically as you shown.