Hi --
I've been meaning to compare Genos vs. MODX, so it made sense to draw up a few lists.
Genos is definitely a step up from S970. The "Real Distortion" DSP effects are Yamaha's latest and greatest. Genos has more Real Distortion effects than S970:
Genos S970
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Tweed Guy
Boutique DC
Y-Amp
Distomp
80s Small Box
US Combo US Combo
Jazz Combo Jazz Combo
US High Gain US High Gain
British Lead British Lead
Multi FX Multi FX
Small Stereo Dist Small Stereo Dist
British Combo British Combo
British Legend British Legend
Some Genos voices use the existing S970 effects with different settings/presets. The new SX900 is at the same level as S970.
Other recent effects include the Virtual Circuit Modeling (VCM) effects which model vintage pedals and such:
Uni Comp
VCM Compressor VCM Compressor
VCM Flanger VCM Flanger
VCM Phaser VCM Phaser
VCM Auto Wah VCM Auto Wah
VCM Touch Wah VCM Touch Wah
VCM Pedal Wah VCM Pedal Wah
Genos has a new compressor algorithm (Uni Comp) that is used extensively in its preset styles. Uni Comp has a number of guitar and bass presets. The SX900 does not have Uni Comp.
Genos has newer guitar waveforms (samples) than S970. In particular, Genos has a Telecaster, Stratocaster and Music Man Sting Ray bass. The Genos voices using those samples begin with "50s", "60s" and "ActiveBass", respectively. The SX900 adds the Telecaster (50s), but not the Stratocaster (60s) or Sting Ray.
The list of Mega Voices, by the way, is a good place to look for new guitar and bass samples. These same waveforms are used in Super Articulation voices. It's a quick way to make a comparison.
Hope this helps out -- pj