I've recently had the external antenna completely removed, the roof filled and resprayed and a high power internal antenna/amplifier installed.

I currently have the standard V8V stereo (not the premium). I find it's actually not too bad but of course could be way better.

Any ideas on how to upgrade the sound?

Am i right in assuming the speakers behind the seats (at the seatbelt tops) are empty? So first port of call should be to upgrade/add better speakers where necessary?
And change the sub for something like a JBL?

I'm guessing an attempt adding a premium headunit is not a clever option?

 

I'm no stereo expert, but I would think the best bang for the buck would be investing in new speakers and amplifiers. The speakers are what ultimately produce the sound and they need adequate power to produce it at higher volume levels. I assume that upgrading head unit to the "premium" unit would be expensive.