Absolutely you can use a 170 as a center - that is why they sell singles. These guys don't really handle a lot of bass though. They go low, but not low enough to me, and long term I wouldn't use them without a sub. If the plan is to add a sub, then by all means...
If you want more low end for the money IMO, the Encore X-LS from
www.av123.com should do the trick. They also have towers that aren't much more.