A PA amp will normally accept +4dBu output, balanced or unbalanced. Some have a sensitivity switch to adjust the line level, often for mixing in a mic without the need for a separate mic preamp. But the output level of keyboards is all over the map. The Jupiter-50 is spec'd at balanced +4dBu, but the Juno Stage is not. Roland doesn't publish the output spec for this keyboard, and Roland customer support specifically refuses to provide it. Grrr.
I believe it likely is -10dBV or close to that as a default. It definitely is unbalanced. The manual does say you can amplify the JS with a home stereo, which is highly suggestive of -10dBV output as a hotter output can damage a home stereo amp. To run this to a PA amp, you do need a preamp in between to get the output volume from the PA you would expect at a given gain setting. There are a number of ways to do this:
1. Use a small keyboard mixer. This will enable you to use a multi-keyboard setup and not only mix the results before routing to a PA, but the mixer will have a preamp for each channel as needed. This also is used to standardize the line level of a stack of keyboards with disparate output levels.
2. Use something like this:
http://www.sweetwater.com/store/detail/LLS2?adpos=1o1&creative=54989263441&device=t&matchtype=&network=g&gclid=CJm_ieTD68ACFUiPfgodxLAAGQ
to convert unbalanced -10dBV to +4dBu output. This device is passive, so no extra power cords needed.
3. Use a dedicated keyboard amp instead of a PA. These have pre amps built-in to support a wide range of keyboard line levels, even lower than -10dBV typically because some analog synths and vintage keyboards have fairly low output.
4. Set the Juno Stage System setting for output level higher. It can be set as high as +12dB. The difference between pro line level +4dBu and consumer line level -10dBV is 11.79 dB, further evidence that the default setting is -10dBV, and setting it to +12dB is changing the output to +4dBu pro line level (still unbalanced). While this may sound like the optimal choice, I've heard that there is distortion when set at +12 dB (I've not tried it). If this is true, it would suggest that the keyboard preamp doesn't have enough headroom to do this without clipping, which would make this the least desirable solution.
This is not unique to the Juno Stage, but is quite common, ie having output too low to get full volume out of a PA amp.
If you need to run a long wire to the PA, option 2 is good because it will be a long balanced connection, assuming your PA amp supports balanced connections. Option 1 is convenient if you have multiple keyboards in your setup. Option 3 has the convenience of an all-in-one package solution, and option 4 will require testing to see if you can get enough output without distortion (which might be true with less of a boost than +12dB also).