The backline 600 is a great amp for the price. However, I would look at Avatar or Aguilar for cabs. I use the Avatar B410 with the Backline 600 at church and I think it's awesome. The GK cabs leave something to be desired. After using the Avatar cab, I went back to Guitar Center to try out the GK cabs and there's just no comparison.
Definitely look at Avatar or Aguilar. Be sure to check out eBay or possibly craigslist for deals on used ones.