I sold my pair of Nano 300 speakers and bought the Nano 600 over a year ago. I think the sound quality of the two small speakers
in the 600 are superior to those of the 300. Of course you have
more power in the amps. and the speakers are "baffled" so the sound is spread over a wide spectrum.
Both the 300 and the 600 are fine units. Bought both the 300 and the 600 from Frank at Audioworks.ct Lloyd
I haven't heard the 600 yet, but considering the construction of the top speakers with 4.5" plus 1" and 80 Watts vs. 35, I would also expect their sound quality to be better.
One thing I find a bit questionable about both 300 and 600 is that the subwoofer goes up to 190 Hertz, where the top speakers take over. So, in the 2.1 configuration, there is no stereo below 190 Hz. In practice however, this hasn't been very striking to me yet.