yeb, the audiolab and Meridian combo seem to be lacking that excitement needed in midrange system. If you meridian is a 24 bit version I would keep it and replace your 8000a with Densen B110 or Creek Destiny. (around £1200) As I don't know your taste in music the 2 amps I sugest are quite an all rounder, will play most music without fuss.
I'd say the speakers then the amp are the ones to upgrade first. The 506 is a classic. Don't know about the audiolab. If you can replace both the amp and the speakers in one go, that will make it easier to audition - just use a meridian dealer such as KJ West One if you're in london. If you like acoustic music, try sugden amps for gorgeous, lush, clean sound.