Co-Op Guitars
Here is how you can unlock additional Basses in Co-Op mode:
Gibson SG Bass - Beat 10 Songs
Gibson Grabber - Beat 20 Songs
Gibson Thunderbird - Beat 30 Songs
Gibson Les Paul and skin - Beat 40 Songs
Hofner Bass - Beat every song
Lava Pearl Musicman Stingray - Get 5 stars on at least 10 songs
Cream SG - Get 5 stars on 20 songs
Natural Sunburst Gibson Thunderbird Bass - Get 5 stars on every co-op mode song (stacks with below)
Natural Maple Gibson Grabber Bass - Get 4 stars on all co-op mode songs