For me the sounds of the PSR-550 were too "soft". I felt they lacked "punch". I was very disappointed with the sax sounds on the 550. The only exception was the sweet tenor, but that sound was for slower type solos. The other saxes were just too "mushy" especially in the lower registers. I was also disappointed with the bass guitars as well as electric distorted guitars.
The piano was decent, but again I felt was very weak in the lower register. The drums were VERY disappointing. This wasn't just a problem islolated to the 550, but muih of the PSR line up.
The newer WK series are (in my opinion) well above the 350/450, and the discontinued 550. The power behind the WK is not only in the quality of samples but also in the effects section. The WK allows you to really dig deep into an effect and customize it. With the 350/450/550 all you can do is set the basic reverb, chorus, and DSP return level (which is global).
The WK has numerous parameters you can adjust within the effects. Plus the effects are really good (especially when you consider the price of the keyboard). I still have yet to hear an arranger keyboard produce the distorted wha effects found in the WK's. The PSR-2000 couldn't even produce this sound.
Plus with the WK's you can edit the voices. Be it basic synth paramters, it is still very nice to have the ability to do so.
In the end I think the WK's are more keyboard for the money. I think the PSR-450 is over priced. I played the 450 on several occasions and each time left unsatisfied.
Squeak
_________________________
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.