Best improvement for the buck

You could explore variety in obtaining a second, different type of speaker to have a contrast. It's a great way to play the game.  

Obviously, you can upgrade your sound with the speakers you have, or new ones, but as an option to hear more variety, a second speaker, such as a horn hybrid, is a terrifically fun way to go. Imo, there are not many things in the hobby more fun than swapping out a speaker and having a "new" rig, a new sound at your whim.  :) 

Full loom of Raven cable, or even better Audio Envy.

I would start with your amp/preamp combo and go from there. Move to source improvements afterward.

Oz

IMO a mediocre horn speaker at $900 new or used would not improve his system, since that is what the OP is after.. A properly set up Magnepan 1.7i rivals box speakers costing ??? you know the rest!

The point of variety is variety. 
Horn speaker was an example. 
Maggie's are all that to some. To others like myself they are just a flavor.