I've also got a 3000.

I had a good play with a 950 a few months back. It sounds immediately better and the SA voices are usable and significantly better.

However the s750 has a lot of the same stuff and is also worth a look. Is the s950 worth the very significant hike over the S750? This depends entirely on whether you need the additional stuff. There are some extra voices and styles, and clearly if you use the vocal processing your choice has been made! The mp3 playback and text file capability are useful but to me not game changers.

The 3k remains an excellent keyboard but time has moved on and the s950/750 are definately superior.

As others will probably say, ultimately you need to find a 950/750, have a seriously long try out using your own headphones, and then see whether you can justify the expense.