Thanks for the link Mike
For me, a big difference is (quote from Bonner's)
The PSR-SX900 has a 50w sound system (15w + 10w x 2) for a MASSIVE SOUND! The PSR-SX700's sound system is total 30watts (15w x 2). The PSR-SX900 speaker configuration is slightly different with the use of dome speakers for a fuller, enhanced sound.
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Larry "Hawk"
♫ 🎹🎹 ♫ SX-900