I've heard a pair of ATC SCM50's driven by a Lyngdorf 2170 as part of a dealer demonstration using a Yamaha S3000 CD player as a source.
The sound was highly detailed, clear, non-fatiguing, open and yes, neutral. All you heard was the music, not the speakers.
However, I found it to be a sound I admired rather than loved, one I respected rather than desired. And they definitely need a Subwoofer (or two) for Bass.
The other speakers I heard that day were the Bowers & Wilkins 804 D4 (which had much better Bass and were more dynamic, but dominated by the Diamond tweeter and quite fatiguing), and the Spendor D7.2 (which were wonderful and had the best qualities of the other two speakers - the speed and Bass of the B&W + the openness and detail of the ATC).
If you're looking for analysis then go ATC, but if you're looking for musical joy I'd recommend you add the Spendor D7.2 to your list.
Personally, I think the Spendor (D series if you can afford it, A series if you can't) would match better with your NAD/Marantz/Technics system and would be a huge jump in quality from your existing Tannoy's. I think you'd be more familiar and comfortable with their sound too, but get a dealer demo if you can so you can decide for yourself with your own music.
In my room, I'd have to disagree on the preference between the Spendor D series and ATC. Yes, you need a sub or two with the ATCs (at least for my SCM19s and smaller), but I found that the D7s that I owned prior to the ATCs also needed a sub for support at times. For me, the D7 could be overly analytical at times. Note I'm talking about the D7 Mk1 as that's what I owned. I have heard a demo of the 7.2s, but didn't think they sounded that much different, but it wasn't in my room or driven by my amp.