Will the center be sitting on top of a TV? If so, I would go for the 340c since it has the EXBAC circuitry which compensates for the baffle the TV face creates. Also, the classic 170's and classic 340c I heard were very well timbre matched, so I do not think there will be an issue.
On the otherhand, a lot is to be said for using the same speakers all the way around.
If you were to go with 170SE's, I would also consider upgrading the classics to SE's as well. Heck, I think I would even if you went with the 340SE.
After reading what I wrote, it does not seem like much help. Sorry.