Ok Mike, here’s my studied opinion. I spent some time reading up on the Zu’s just to be sure. Your speakers which are about 10 years old, have a MK2 upgrade all new drivers $799.00 but of course out of stock. The Zu speakers are super efficient 99db 1watt@1meter thats really efficient. Zu speakers are a favorite of the SET, and other low power Triodes, tube crowd due to their efficiency. What I find peculiar is this On and Off SQ problem; some times it On sometimes it goes Off. I can honestly say in 45+ years of audiophile -ness I’ve never had that problem. Gear works or it’s broken. If there’s a short it’s usually fairly obvious.
That said I’d try a couple of other simple things are the drivers All snugged down? take a whatever tool and check, don’t worry about exact torque, just make sure they are all similarly tight, not Gorilla tight, just tight.
Are you using any Mains Conditioning/ Filtering? My best guess is that there’s things on the AC Mains that causes your system to be sonically On then Off… I honestly couldn’t imagine not using a good PLC. Naim doesn’t like them but I believe that’s wrong and based on old thinking that PLC’s will limit current. And 30 years ago they may have been right. But with modern units I believe they are wrong. We have so much more crap on the Mains compared to Years ago a PLC is almost mandatory. Shunyata, PS Audio, Audience, EVO3, Synergistic Research, AudioQuest, Furutech. Lots of really good ones to choose from, that do not limit current. And yes they do change the sound. Typically reducing noise and hash on the AC Mains leading to much quieter background increased Blackness.
And lastly I also think Naim may not be the best choice for the Zu’s… If you are were going to keep the Zu’s and you’re not going to use tubes ( which would be the best , but would make you a bit batty) then The best choice IMHO would be Pass Labs. I’d recommend the INT- 60. A lot less fussing and impeccable sound. The INT-60’s first 30 watts are pure class A then it switches to A/B but it’s all about current into a load. Fast, transparent, warm-ish, natural…Lots of great reviews, and bullet proof reliable.
I’m going to bow out now. But I think the current road you’re on is just going to keep getting you pis**d off . I only have 45 years of experience, selling, setting stuff up, buying , trading, customer service, doing Shows,CES etc… I believe having all these boxes is just too much for you. Get a great a single box like the Pass you will be a Happy Boy!