I've had the original eons(10s AND 15s) and I now use the G2 Tens, and I can tell you - the G2 is more than just a volume improvement. The tone is much better, the bass is excellent (although for really pumping dance bottom, you NEED a sub)and the size/weight ratio is fantastic.
The eon15s are cheap because the new ones are such an improvement. They are well worth $399, but if you can swing it - the G2s will perform better.