Wow - we complain about heavy kbs all the time, but I NEVER thought of the kn6000 as a beast. That's not heavy at all, compared to most of the rest. The reason the 740 is so light is that the speakers are so underpowered. (basically useless, out of the home)
The newer psr2000 is a much better bet if you play out - better speakers, better mic input, same light weight, but a beefier system, and they give you two sets of outputs on it. One is fixed volume, and the other follows the knob. I'd check it out b4 you get the 740 - it's a BIG improvement.