The spendors are not very efficient and the Nait is not very powerful, so I can only imagine that the combination will work in a small room, though it would probably work well.
I have a friend in the UK who has a 15 yr old Quad pre/power combination (I think 100w/channel) driving ls3/5s and the Quad's unusual tone controls (tilt/lift) are very useful for giving the bass a bit of a lift on thin recordings. I know tone controls are usually a dirty word, but they can really help the ls3/5s with their limited bass. I've seen used quad pre/power amps for anything from $600 on agon.
Lastly I have a densen B-100, which is similar to the naim, but with more power. I have used spendor3/5 (borrowed from a friend) and it sounded pretty good. The densen is about $1300 new, from audio outlet in NY.