Personally i wouldnt use the CNT with a 600 series speaker. U'll find the cente speaker will constantly upstage the fronts. it is a lot more detailed and has far greater midrange projection than the 600 series as well as a surprising amount of bass grunt for its size.
The CNT has even greater bass than the 1NT's probably down to a slightly larger cabinet volume and the use of 2 flowports.
The best match for the CNT is at least 7NT's the 1NT's will match but the centre will still end up with a little more bass weight to it. But if budget is a constraint then the 1NT's will definately sound a heap better than the 600 series.
I think u'll be impressed with the CNT... i was