Sean's info is correct as far as it goes. All the different versions sound different and probably shouldn't be mixed in a bi-amp situation. The 70 wpc and the first 2 versions of the 105 wpc amps look identical on the outside with gold lettering and silver handles. To identify them you will have to pop the lid and take a look inside. The last version is the one with red and blue lettering and is a completely different design. As Sean said it uses different output devices. While still rated at 105 wpc it uses 2 pair per channel as opposed to the earlier use of 1 pair per side. The circuit boards and internal layout are also different. Because of it's use of more outputs the newer design does not have the problem of insufficient current delivery into low impedence speakers that the early 105 wpc units did. Sonically they are similar but not identical. The older amps have a very sweet & palpable midrange with a slightly loose low end and somewhat unrefined top end. The newer version has most of the midrange magic with a much tighter bottom and sweeter top end. I own an early and a late 105 wpc version and have compared them extensively. They are both really nice sounding amps but in the long run I prefer the newer red and blue version slightly on most music. You wouldn't go wrong on either one though as this amp is the all time greatest giant killer bargain amp. Here is a partial list of amps that have been bettered by the little B&K in my system. Aragon 2004, Hafler Transnova 9300, Meitner STR-50, Sumo Andromada, Counterpoint SA-12, Denon 2200, ASL 1003, and several others I don't recall at the moment.
