I dont think the base stereo is sold in the us, but this list from the disco3.co.uk covers just about every difference:
Low Line (Base Spec) which comprises
low line head unit with a built in 4 channel amp delivering unknown wattage per channel.
6 speakers in total made up of;
2 mid/bass in front doors,
2 tweeters in front doors
2 full range in rear doors.
wired with;
1 pair of wires going to each door (4 channels).
Hi Line which comprises
High line head unit, 6 CD changer
Separate 6 channel Amp (AUD2C) under seat delivering 50 watts per channel
9 Speakers in total
2 mid/bass in front doors,
2 tweeters in front doors,
2 mid/bass in rear doors.
2 tweeters in rear doors,
1 Sub woofer in tailgate.
wired with;
1 pair of wires (channel) going to each door (4 channels).
2 pairs of wires going to the sub Woofer (2 channels)
Premium which comprises;
High line head unit, 6 CD changer
Separate 12 channel Amp (AUD4) under seat delivering 50 watts per channel
14 Speakers in total made up of
2 tweeters in front doors,
2 mid in front doors,
2 bass in front doors,
2 tweeters in rear doors,
2 mid/bass in rear doors.
1 Sub woofer in tailgate.
2 surround speakers in each rear pillar
1 center speaker in middle of dash
wired with;
2 pairs of wires going to each front door (4 channels)
1 pair of wires (channel) going to each rear door. (2 channels)
2 pairs of wires (channels) going to the sub Woofer (2 channels)
1 pair of wires going to each rear pillar speaker (2 channels)
1 pair of wires going to the center of dash speaker (1 channel)