Hi Nick
I keep seeing this thread title, and for some reason thought I'd already replied earlier in the thread! But I notice I haven't. So here goes...
The Nait 5i (i or i?) is a cracking little amp, and as much as I love it, it does have limitations. Many speakers already mentioned just aren't going to be controlled properly with the 5i, which will lead to a dull, lifeless sound. Seeing as you like to drive things hard every now and again, I'd stick to higher sensitivity speakers, and preferably easier loads as far as impedance are concerned. The Neats, which are designed on Naim equipment, would be the first port of call. A nice lively sounding speaker that isn't going to demand too much from the amplifier. The new Motive 2se would be the ideal model here, especially if higher volumes are called for.
KEF's new Q900 will also be a contender. I've been trying out the Q300's on the Nait and they're fabulous. The only drawback here is that Q900's are going to be a couple of weeks away.
B&W's and Dynaudios need a bit of space to work well, and need more bass control than the average speaker, partly because their tonal balance is shifted more towards bass. It's not that these speakers outclass the Nait, it's just that the Nait, and many other amplifiers in the Nait's price bracket just don't have enough control for them.
ATC need more than the Nait 5i can provide. Their low sensitivity means the volume control will be up a couple of extra notches, so if you already listen close to 12 o'clock, the Nait will run out of steam. ATC's ideally need at least an XS to start getting what's required from them.
ProAc Studio 140.2's wil be worth trying - a nice energetic sounding speaker that should work fine with the Nait 5i. The Response D18 ideally needs a Nait pre/power or a SuperNait to start getting the best from them.
We felt that when Tannoy raised their prices, the Revolution series sort of priced itself out of the market, as it brought the DC6T up to the KEF XQ30 price point. The little KEF's are an amazing speaker, and if you like their revealing, 'not scared to give it to you' nature, you may love them, so they'll be worth a try. They're not hard to drive, so should work well with a Nait 5i.
All of the above I've commented on (other than the Q900, and the Motive 2se's - only heard Motive 2's) I've heard with various amps here, including the Nait 5i.