Can I use Roland Cube 15 guitar amplifier with my Yamaha P80 digital piano?
The current model of the Roland Cube 15 is one of a new generation of guitar amplifiers which can also accommodate a signal from a digital piano.
The Input jack is a 1/4-inch phone plug jack that is designed only for the signal from an electric guitar. This is a signal that is low in volume and high in impedance. Do not connect the signal from your digital piano to this jack. If you try to connect the output from your digital piano to the "Input" jack, you will get a distorted, very noisy sound that will not be satisfactory.
Rather, the Aux In jack is a stereo 1/4-inch jack that is designed for the signal from an MP3 player or iPhone. This is the jack to which you should connect the output from your digital piano.
Make the connection with the Yamaha P80's L and R RCA-style jacks under "Output" as shown in the picture above.
In order to connect the output of your digital piano to the Aux In jack, you will need to find an appropriate adapter cable. According to what I have read in the Roland Cube 15 owners manual, you need an RCA stereo to 1/4-inch stereo adapter cable that looks like this:
You should be able to find a suitable adapter cable at any music store.
If you want clean sound for your amp, I suggest to buy a keyboard amp.
Guitar amp is natural sound, they have no limiter/noise filter. You can plug your keyboard into your guitar amp. But, it'll make some noises.
For Roland Cube 15W, don't play the keyboard in maximum volume, so, the speaker won't blow / pop.
You CAN. But piano is a very dynamic sound, and can easily distort on under-powered amps and under-sized speakers. You'll be a lot happier with something more beefy.