Flat is essentially off - all EQ settings are at zero.

Personally, I've never had a problem with the bass on any Yamaha keyboard I've owned in two decades.

Gary cool
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)