What Tuga said .. you really need to get out there and listen. I bought a Nord amp and it was ok but I did not like it as much as my current amp. Your Harbeths are fairly inefficient (86db) so the beautiful Sugden A21 would run out of puff,That is a question that only you can answer...
Do you feel like a change?
Do you expect the Luxman will be an improvement on your current Linn system?
I would at least try to listen if possible.
Contact local wammers perhaps and go listen to theirs, if you fancy trying a Hypex based Class D @orangeart (a member in here) sells his own amp and does allow for home dems with no obligation I believe (although expect to pay carriage to and from you if he is prepared to do that for you). Get out and annoy the dealers and see what they have and if anything takes your fancy.
The Lux amp is going to set you back about £4k and there are a lot of class amps out there for that kind of money (and second hand I can think of quite a few that would tempt me (I would go separate pre power but that is another story). I think you owe it to yourself to try the higher powered Sudgens and the veritable Prima Luna (although the power rating may be just a tad on the low side depending on how loud you listen and the size of your room.