No way you can compare the 500 series to the 800/900. I played the 800 for 4 years, 500 x a year. I bought the 500 for a backup and it was like a toy in comparison, thus the reason it doesn't sell well.
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Live: Korg PA4X/Bose L1 Pro 8/Senn 935
Studio: Korg PA4X/Kawai MP7SE/Tascam DP24SD/Sony C80/AGK 214/ATEM Mini Pro switcher/