Yeah i thought the stock jet city speakers were a bit tinny- however, I already had the v12s ready to swap, so I only gave the stock jet city speakers a cursory try before I swapped them, so they obviously were nowhere near broken-in and may well have improved as time went on. However, neither were the V12s and out of the box I thought they sounded noticeably better. Not a totally massive difference (at least at the volumes I was playing at), but yeah, better.
I haven't really tried much combined with a V12, so I can't help you there- the ones I have are a different impedance from most of my other speakers (because when I bought mine you couldn't get the 16 ohm versions retail). I only tried a V12 with a GB128, and I didn't really like the mix, I preferred the two V12s on their own. (I like the GB128 too, sort of like a more modern-sounding, cleaner greenback, just not combined with the V12!).