My first recommendation would be to just keep enjoying what you have - if it ain't broke, don't fix it. But if you simply must upgrade then...
A while ago, I had the kimik upgrade done to my SRM-007t mk1. The upgrade was worth the proportional extra cost, but no night-and-day change, just a bit more of everything that was the original. I imagine the same would apply to your SRM-006t mk2.
Some say, including me, that the 404 sig has an upper mid peak, which got on my nerves after a while, so I upgraded to O2s, which were a massive improvement. However, some may actually like the extra presence that that gives, so you need to make up your own mind. I haven't heard the 507, but from other posts, I would say that the 507 would give a bigger improvement than the kimik mod.
Now go read that big Stax thread to find more answers than you could possibly have imagined 
Edited by TheAttorney - 6/7/11 at 4:53am
