A21SE vs Supernait 3 vs Eci 6 mkII

You’re going to have to hear them, each with sympathetic speakers. What suits a Sugden probably won’t suit a Nait, and Focal may well favour the Sugden despite the common ownership with Naim, I don’t know the ECI.

I haven’t spent any time with the ECI but I’ve owned Naim Naits previous and demoed The Supernait 3. Currently I own a Sugden IA4 which was an upgrade from an A21SE. Agree with Yeti that the proof is hearing it in your system but I can speak to them a little generally. 
I would say the Naim gear sounded better with more typically sensitive speakers - 89db or less to my ear.  I’ve heard them sound excellent with Harbeth, Falcon and Graham Audio for example. Didn’t like Naim as well with DeVore, Klipsch or JBL. The Sugden amps have served me well on Klipsch Heresy IV, ATC 40 and now JBL 4367 speakers. I’m definitely a fan of the class A sound. 

Thanks for the input guys. Yes I agree listening and comparing them side by side is the best. But from where I am, I'm unable to do that. I can only listen to them in different shops.  And from a price point, the Supernait 3 is closer in price to a Sugden Anv 50, IA4 are about 1000 usd more. 

The A21SE is tremendous so if you can audition absolutely do. I went to it from a Luxman 507ux which was also a great amp - after a side by side I could give up the Sugden sound so the Luxman was sold on. When I had the Supernait in house with the A21SE within a song or two I knew the Naim wasn’t staying. 

Thanks Kevin, that's good to hear. I'll save quite a lot of I do go for A21SE. My only problem with the Supernait is that it sounds a bit cold sometimes but it's very detailed. I've listened to the A21 which sounds more musical but the bass isn't as clean. Hopefully the SE sounds better. 

The A21se hands down....you will love it, as I do. No problem with bass here...

The A21se sounded better to me than the Supernait 3 with a "faster", more articulate reproduction of music as well as a fuller, more upfront sound. The one area in which I thought the Supernait did better was the ability to play louder or provide more power for less sensitive speakers

As usual it will depend on the speakers that will be matched to the amp. The Sugden A21SE will sound slow and lack the pace, rhythm and dynamics of the Naim Supernait when matched with certain speakers.

@ryder I agree with you. I finally heard the A21se with magicos and it sounds slow however, I listened to the Supernait 3 with Focal and they seemed a lot faster and more agile. For me, the Supernait seems to be easier to match

It's a little unfair to compare Luxman 507 with Sugden. A better comparison will be Luxman 550 or 590 which run in class A much longer.

OP -- you should definitely put the Luxman 550 on your list. If your budget permits, a 590 AXII is a solid contender.