Better to use the 9V 800mA one (if it is DC and correct polarity):
The current rating (amps/watts) is how much the adapter can supply without burning itself out. The device being powered (MT-32 in this case) will try to draw whatever it needs, which according to your info is no more than 650mA. In other words, devices pull power from power supplies - the adapter doesn't push power to the device.
Mismatched voltage, on the other hand, can cause damage to the device being powered because its circuitry is designed to run off of a specific voltage range. Even voltage regulators can only accept a certain range without blowing up, so you can't count on those. 10V may be close enough to not cause problems, but why take the chance when you have a 9V supply?
Realistically you'll probably be safe either way.