Just posting a small heads-up after 2 weeks of frustration : The Harman Kardon Citation One speakers don't work well when use in group or in speaker pair combined with a Deco M9Plus. Tried all possible configurations, the sound will always end-up crackled. Probably due to the sync-signal between the speakers in conflict with the wifi mesh signal. Though not confirmed, I suspect the entire Citation line will have this issue. I don't have the issue with other routers (old or new) so I wonder what the kind of implementation tp-link cooked up that could cause this.

Contacted both support departments, who are aware of the issue but blame the other party with vague accusations. They don't have a solution available and I don't see a lot of signs they are willing to find & fix the root cause. Even worse, they are not willing to warn potential buyers that the products are not compatible. While the HK remains friendly, TP-Link support was outright rude (stating amongst others it is up to the customer to figure out it products are not compatible!). For a premium product, I expect premium service. Did feel I was getting this. Hence I decided to move to Ubiquiti Unify - no issues, more control, more flexibility...