Vern,
First and foremost, the EQ settings of the PSR-3000 are critical, even when using the Bose PAS system. Every sound system is different and requires its own EQ settings.
Next, what volume level were you running the Z-5500 at? For a room that size I have been running the system at Boost 5 to Boost 6 on average, and that's usually more than enough volume with the keyboard's main volume set at about the 1 O'Clock position.
One thing you must consider is that you are not working with a conventional sound system and monster speakers. Consequently there is a tendency to believe you are not reaching your audience with sufficient volume levels, however, this is usually not the case. I, like many other performers, thought the same thing when I first tried out the Bose PAS, which again, is not a conventional PA system. In both cases the volume levels were more than sufficient and increasing the volume usually resulted in blasting the audience, which is not what I or anyone in this business should do.
The best advice I have is do not be hasty is selling the Z-5500. Instead, get an opinion from your audiences, and from your spouse--both of which will have no qualms about telling you if they like or dislike what they're hearing. (Other musicians are too opinionated, especially old guys like me!) If I had the time I would hop on a plane and head west to help, but from now till after Christmas I'm booked solid.
I gotta' go sailin',
Gary
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Travlin' Easy