Went up to my local Maplins store today and picked up a 12v AC 500ma power brick. Plugged it in and hooked it all up to my system. After a couple of hours, first impressions are excellent - certainly more 'air', 'space' and detail than the NAD. Bass seems to be about the same or possibly a touch less, but that isn't a bad thing on some recordings that seem to have too much bass and thus sound a little distorted.
Had a look at the other power brick the other night that was causing problems - it was outputting 300ma - that might explain why I was having trouble!