Yves,
If you have a WK6 you can easily add a vocal harmonizer inside the keyboard and also a hard disk drive. The hard disk drive requires a 2.5"ide type which is available from $35 to $75.00 for a 540 megabyte refurbished drive or a little more if you want a bigger one, and the hard disk mounting kit runs around $200.00 because it also includes a plug in module which is needed to add ram so the hard drive will function. The drive is available from many mailorder places and the hard disk kit is available from Generalmusic. The Vocal harmonizer runs about $400.00 and also mounts inside the keyboard. It is very easy to install both of these options. If you want a new keyboard, than get one, but if you want to upgrade your wK6 you can do this as well.
George Kaye