I don't play in any band. I'm strictly an amateur enthusiast but I know my electronics pretty well.
If you look around there are other brands of powered studio Nearfield monitors out there ranging anywhere from under $200 a pair to well over a thousand.
The M-Audios I used for years without a subwoofer and they were only 5" woofers.
The thing is, I really didn't realize what I was missing from the sound until one day I shelled out $400 for the Mackie Sub.
The difference I'm talking about here applies to any audiophile system these days. Sound is very subjective and depends on what sort of music you play and listen to.
If you're into prog rock and metal then you're going to want as much room shaking bass as you can get. If all you are looking for is to practice your keyboard skills without breaking the bank then a pair of 6" woofers is adequate for the job.
Then again there are some real cheapie solutions that will work just as well.
Since a keyboard's frequency range and dynamics are roughly 20Hz to 20KHz pretty much any decent set of computer gaming speakers or a good quality audiophile home stereo will do in a pinch.
To hook up to your stereo's aux input all you need is a proper cable like this one.
http://www.amazon.com/Hosa-Cable-CPR203-Dual-Inch/dp/B000068O18
If you hook it up thru the tape loop with a small mixer you'd even be able to play music from other sources while playing along on your home stereo.
Or a decent set of computer speakers that would do the job are these Klipsch 2.1 gamer speakers simply using the right cables and adapters.
http://www.amazon.com/Klipsch-ProMedia-Certified-Computer-Speaker/dp/B000062VUO
The main point is that these options are all electrically and acoustically compatible with your keyboard.
These wouldn't be giggable options but hopefully the house would supply a built in PA system anyway and all you'd need is a good mixer and a few cables for a professional gig.
Gary