Although the bridged output of the zx350.4 is 175w RMS, this is for a minimum of 4ohms. The KTM6's actually ohm out at a little less then the stated 4ohm. This may stress the amp a little in bridge mode, but I've had one running since May with no problems.
The zx350.4 is cheaper with a little more power when bridged, but the zx450.2 will be more stable and run a little cooler. Wont gain anything by running the zx450.2 mono as the 2 speakers would split the 225w's for a net of 112w, besides the impedance being to low if run in parallel.
If you really wanted to get the most out of a set of KTM6's, drive them with the zx550.2 (205w RMS x 2)