I proudly own a 309 Polished Ebony, still an advanced keyboard (just mention the two USB input, aux in and out, and others convenient features).
After a long study and search, both on the web and visiting many music stores, I finally bought a pair of Yamaha Hs8 powered monitors.
The result is a completely different instrument!
It is not a matter of adding volume, but of reaching a sound that the internal speaker could never deliver. In facts, I "discovered" the real sounds of CVP.
I'm using the monitors TOGETHER with the CVP speaker system, with the following balance: 1/3 CVP, 2/3 Hs8. And the result is amazing.
I had considered the smaller Hs7 also, but the low frequencies warmness is completely better on Hs8.
Also, on Hs8 is amazing how you can distinguish very clearly each voices when playing styles.
If you compare the price of a CVP 9 and the price of a pair of Hs8 (I paid 580 Euro), and you consider what an improvement, this is an excellent purchase.
In order to get the best effect, I also ordered a pair of very study Konig & Meyer stands for the monitors, to have the monitors tweeters at the same height of my ears (as suggested by the Hs8 User Manual).
Big recomendation to CVP owners.
Ciao