An interesting twist on the Stage Piano from Casio - with the Privia Pro PX-5S you get the 88 Key weighted keyboard - made from some sort of plastic that doesn't have the shine of an ivory feel, but certainly seems to play well and weighs a LOT less - at 24.47lbs so easily luggable - in fact I suspect your keyboard stand weighs almost the same.
Under the hood though we get a supercharged DSP and PCM sound set (AiR Sound Source) which gives you a reasonable piano and a ton of very playable electro mechanical patches.
In addition, there's the Hex Layer synth engine - this is what we got in the Casio XW-P1 too - each voice has up to 6 single oscillator layers - each with it's own multiwave source, multimode filter, pitch amp and filter envelopes too. Each layer can call upon up to 400 PCM waves including multisampled sound sets and plenty of synth waves. Stack em up and let them go!
In practice, you can create some pretty cool, complex voices - even without PWM and OSC sync.
Power With Control
The third major component is the MIDI Controller aspect - at the top level there are 99 stage patches - each of these has up to four Zones (key range, velocity split, or just layer). These can fire internal voices, external MIDI or external USB devices. There are also four assignable knobs, 6 faders, 2 footswitches too - these can all be routed to any zone/MIDI output. Sadly, there's no aftertouch though.
Setting up Zones is pretty simple to do, just a few button presses and browsing the voice categories (Piano, Electric Piano, Organ, Strings/Brass, Guitar/Bass, Synth/Various, Drums, Hex Layer).
But deeper programming is where you get a bit bogged down in menu diving and repeated button pressing - it's all a bit 1990 to be honest and the unfamiliar nature of the Casio OS adds a little extra to the learning curve. Fortunately there's an editor for the Mac and PC, though it's not going to win any UI design awards, it's functional and certainly helps.
What is impressive though is the overall breadth of patches. Granted, perhaps the weakest point is the actual acoustic piano - which is a little disappointing seeing as it's a supposed to be a Stage Piano. But the sheer lushness that the 256 voice polyphony can deliver is not to be sniffed at.
The effects add a lot too with 4 simultaneous insert effects (Equalizer, Compressor, Limiter, Enhancer, Early Reflection, Phaser, Chorus, Flanger, Tremolo, Auto Pan, Rotary, Drive Rotary, LFO Wah, Auto Wah, Distortion (w/ Amp Simulator), Pitch Shifter, Multi Chorus, Ring Mod, Delay, Piano effect), plus System Effects Reverb, Chorus and Delay - there are some wonderful pads and unique synth tones to be had, with plenty of real-time sculpting from the 4 knobs and 6 faders - they can be assigned all over the place so you should see great results if you get stuck in - though labels like DSP param 7 are not all that helpful.
One thing that cannot be overstated is the impact that having an 88 key controller has on your playing, it expands your technique and repertoire and I found that a lot of the sounds were very inspiring to play - the electro-mechanical voices - electric pianos , organs and clavs etc are of the kind that let you just stomp on the sustain pedal (included accessory) and let it ring out. When you start playing with the hex layers - you can have two in a single stage patch (only a mere 12 oscillators per note) it's easy to get lost in sound.
And I guess thats the thing - it's got a bit of an identity crisis - as a Stage Piano I might be looking for a really good basic piano - which the Casio doesn't have - it's okay, but not mind-blowing. As a synthesizer I might want a more featured voice structure (PWM and Sync, maybe a few more modulation options). To compensate, the Privia PX-5S just throws voices at the issue and in many cases succeeds.
It's got a lot of features, but they are quite random in many ways - if you want all of them then it's brilliant value for money at $999, but if you were hoping for more traditional set of sounds from a Stage Piano it might not be for you.