The A5's do seem to need a fair bit of driving for some reason. Their sensitivity isn't spectularly low, but they seem to act like an 83/84dB speaker. As a minimum, I'd say you'll have to be looking at the Audiolab 8200A, but ideally I think you'd need to be looking at something like Arcam's A38. There wouldn't be many amplifiers below the £1k mark that I'd recommend to do the job properly - there's even some up to £2k that I'd not recommend.
If you're looking at budget amplification, you'll need to look at more efficient, less demanding loudspeakers