i guess it's enough if you make extra just the difference between the price of the new keyboard and the money you can sell the old one for. that is making it worthy.
but you're right. that is why i still have and keep the old faithfull Pa50. I believe it is still doing well, sounds good and up-to-date enough so I will skip the actual generation of Pa. Probably I will get the next one though.

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Yamaha S770, Studio One 3, EMU 0404USB, ESI, ATH, Dell. And others.