My audio sales person told me just last week that the new "15" series uses digital amplifiers as opposed to analog in the older series. It comes down to a matter of taste. He feels the analog amps produce a warmer more natural sound in the music. I have two Rotel subwoofers, one a few years old using an analog amplifier/power supply and one I just bought back in Dec using the newer digital amps. I run both at the same time. The newer one (ASW610) seems to produce more thumping than the older ASW600 but that could also be due to room location and the newer design in driver mounting. Hope that helps somewhat.
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