Flex6600 / KPA1500

That is what I do with my 6400 and KPA 500 / KAT 500 combo. I also use DDUtil to set the transmit power based on the status of the amp (operate vs standby).

My KPA 500 has two RS-232 connections, one goes directly into the back of the Flex and the other goes to the PC, under control of DDUtil. The KAT 500 has one RS-232 connection that goes to the PC under control of DDUtil. With this arrangement, I have complete control of the amp and tuner whether I am operating from home or out on the road.

When I am operating remotely, I leave an old laptop running in the shack with DDUtil running and the amp and tuner plugged into it. I then use TeamViewer to control the amp and tuner via DDUtil.