The S950 (and S750/Tyros) all let you do a quick record of a song using a style, and you can easily shut off any style parts you don't want recorded, either before or after recording. You can then add 8 tracks of your own playing on the keyboard to the original recording. The style parts comprise up to 8 tracks of the 16 tracks total available.
As far as looping a bass/drum track to record over (someone correct me if I'm wrong), I don't think it can be done on a Yamaha PSR/Tyros sequencer.
Also, the editing on the Yamaha isn't as powerful as on some competitor's on board sequencers, such as Korg, but it is workable.
To sum up, you can combine aspects of quick record (using a style) and multi-tracking in whatever way suits you best.
I hope that answers your question, Tom.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.