Back in the 90's when I had my night club, I used two keyboards. I had the arranger on top, usually Technics, and a synth underneath. First a DX7, then a V50. Later I added a TX81Z module. I had the bottom keyboard midied to a Yamaha piano module, Each was controlled by a separate volume pedal. That way I had could have piano on call at all times on the bottom. I could bring in strings or organ from the synth, and had the arranger and its sounds on top. Also, until the onboard drums became acceptable, I had the arranger midied to an external drum machine.
Great sound, but not "for the road".
At the time I used two Toa sub bass cabinets and two full-range Peaveys, and a 1000-watt stereo digital power amp for the mains, and another for the subs.
In the rack was a Digitech Vocal Harmonizer, 32-band e.q., Enhancer, Compressor/limiter, two effects units (one for reverb, one for delay). On the other side I had a Roland MV30 for extra midi playback, a Stereo Hi-Fi VCR and a double cassette deck for recording, plus a CD deck for playback during breaks.
In addition I used two full-range powered monitors. Of course the lighting was built-in. You can imagine what all this cost and the all the wires, cables, etc. involved.
Today, the lowly PSR2000/2100, has almost ALL these features incorporated. The Vocal Harmonizer, compressor, effects units, midi player, monitors, modules, drum machine, etc., are all included in the keyboard. It's simply amazing to me.
Throw in a lap-top computer and all the books of lyrics are eliminated, plus many new features that were not even dreamed off 15-20 years ago. MP3s, K******, unlimited styles, Van Basco, . . on and on.
It has become so consolidated that now we are thinking of carrying more gear JUST FOR SHOW!
DonM