Bog Bloke you have a good source there just now, so I would be looking at amp & speakers, you can buy a lot of amplifier for that price range (see ex-dem sticky) but you will have to consider the next upgrade in your system at this point in future, ie your speakers, no point getting a much better amp if speakers are to remain the same. But you could do both at same time :
Ie £400-600 on an amp, my choices would be Primare A20, I20, Unico, Sugden A21.
Then remaining monies on speakers, here its an individualstic choice TBH. But all depedant on what you think your speakers are like at present.
The 309's are an easy load but will not suit a Sugden. They are a bit full sounding and would benefit from a lean and mean amp like the Naim, though I would recommend trying something with more grunt than the Nait 5, perhaps an exposure or maybe a primare... I would bung the ports too to speed up the bass and give a little more punch in the mid.