The main thing really is knowing how loud you're intending to play them. While you say low to medium volume, you're medium volume might be someone's else's high, or someone else's low.
For a room that size, I would usually recommend a speaker with a dual bass driver rather than single. The demands on a single bass driver at higher volumes can be a little taxing, and sharing the work with another driver will ensure that they stay within their designed operating parameters, producing the signal cleanly, and without any added distortion to that that the speaker naturally produces. I'd also try and choose something fairly efficient in order to make the most of the power available from the amplifier.
If you're looking for a big speaker with a big presence that will fill that sort of space easily, have a look at the Dynaudio DM3/7, KEF Q900, B&W 683, Monitor Audio RX8, or Acoustic Energy Radiance 3. But generally, going for the largest speakers you can afford with as many drive units as possible doesn't necessarily get you the best quality, and seeing as you're not going to be playing them at higher volumes means that the aforementioned may be a bit overkill. Plus, I've never felt that B&W or Monitor Audio are a great match with Naim.
Going by my dual driver recommendation, and sticking within budget, I would try the KEF R500. They're efficient, possess a surprising bass response for their size (reaching deeper than some speakers costing another £1k), and hold themselves together well at higher volumes. Assuming your budget is up to £1500, there are two models worth considering just outside of that - the Dynaudio Excite X32 and the ProAc Studio 140se.
Alternatively, if you want to get a little more for your money, you could look at B Grade (imperfect) models which will save you some cash or allow you to get the next model up (or more) for your money.