Feb 28, 2008 05:56 pm < strike that >
EDIT:
Oop, looking at it again, I don't think there is a line level input. the specs say instrument / mic input. Neither of those want to see line level signal, so I'd advise against it.
If you're hard pressed to make it work, you could send line level from your keyboard, but turn down your keyboard output to like 10%. This will send a very low level signal, that may be close to instrument level.
But, I'd still advise against it, as you could overdrive your toneport intput, and permanently damage the preamp circuitry.