RC
The DGX-640 Data List shows Styles 148 through 165 as presets all belonging to the "Pianist" category. It appears that, for some reason, yours are missing. Normally these would be some type of left hand piano comping, either alone or with drum and/or bass. You could try a factory reset in an attempt to bring them back, but you would want to back up any data you may have saved before you do that. If that does not work, then you may have bad firmware. Since the Yamaha site does not list any firmware upgrades, I assume the firmware is in non-rewritable ROM, in which case, you could have a DGX with a defective ROM chip that was not completely populated. I hope that is not the case. If you can come up with a DGX style file somewhere (Yamaha Musicsoft site), you could try loading it to one of those style numbers. If the load takes, then you would know that Yamaha intended those locations as trade-offs for User Styles, and yours somehow got deleted or overwritten with empty files. Good luck !