There are other issues as well. Squeak_D and I were discussing the ROM size on the PSR550 and PSR1000, they are both 8mb, but this was Steve Deming's answer, "Hey guys
The PSR1000 has separate ROM chips for Voice and OS. They are combined in the PSR550. So the PSR1000 has roughly twice the voice ROM of the 550. Hope that clears it up.

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Steve Deming
Assistant Manager
Customer Support Dept.
Pro Audio & Combo Division
Yamaha Corporation of America"