How much does a Shuttle 6.0 go for?
I tried a Shuttle 9.0 a few weeks ago and it was phenomenal.
There are many who like the Shuttle 9.0 even more than the 6.0. There are some long discussions about the 9.0 on TB. Its peak power ratings are higher, and its voicing is somewhat different - all discussed in those threads.
This is a good thread, where in post #17 Agedhorse (a Genz engineer) discusses differences in the Shuttle series models and the Shuttlemax series. http://www.talkbass.com/forum/f15/genz-benz-shuttle-9-0-discussion-thread-594562/
It's a long thread, but Agedhorse was active in it and provided a lot of great info.
For my use, the Shuttle 6.0 has at least 50% more power than I ever expect to need, and I like the sound through the Shuttle cabs, so all is well. I have wondered what it would sound like through a Neo 15 or Neo 2x12, but not enough to spend money on the experiment.