Forget the series 2, even the dual tuner model. With the cable services removing analog channels, the dual tuner model will become a single tuner using only the cable box.
Series 3, HD, HD XL, Premiere, Premiere XL. These are 2 tuner models that use cable cards for cable services, as well as Over the Air.
Premiere 4 and XL4 are 4 tuner models that is only capable to use on digital cable.
Premiere models are Series 4 platform.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!