I have owned Ohms starting with a pair of Ohms F's to the Ohms Walsh 4's now with the Ohm's 300mk11, what can I say I am Hooked on the sound. The 300's will work very well with my large room 20x20x18 feet high. The 300's are power hungry and run at a average of 6.5 ohm's impedence your Mac will push them but not to high levels. I had a Mac 2270 which was replaced with a Bryston 4BST with much better results. As far as the long demo period these speakers require a long break in period nearly the lenght of the home demo period.Ohm also they offer a trade in policy 2nd to none, each time I replaced a older model with a newer one I recieved full credit for the purchace price of the model I was trading in. These speakers will treat you to a wonderfull soundstage great imaging and a non fatiguing sound for years to come.
P. S. your room is not small for the 200's I would recommend them, they also would work well with your 6900
P. S. your room is not small for the 200's I would recommend them, they also would work well with your 6900