Hi!
The JV-1080's piano sounds are OK!
You don't say what you have compared it to, so it is difficult to understand what the problem might be. But as FAEbGBD says, the JV-1080 was released as early as 1994!

Otherwise I think that the problem could be: -that your keyboard (Alesis QS6.1) is sending too low velocity via Midi to the JV-1080, so that the sounds become weak, loose brilliance and don't sound as they should at all.
Try increasing the keyboard sensitivity value (or whatever Alesis calls it!) in the QS6.1, and see if it helps.