The 800 is WAY more powerful than the 500. Fingered on bass is a real problem on the 500, which in itself, is a deal killer for me. No mic input, no EC-5, no aftertouch, very limited extra banks, just to name a few. Depends on what you want.
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Yamaha 920 / Yamaha DGX-670 / Maui 5 Go (2) / Senn e935 / Sony C80 / Tascam DP24sd / ATEM Mini Pro switcher (studio)