Well you really should NOT use a voltage that is higher.....even if it works short term. The DC circuits have caps and diodes designed for a specific rating...if the manufacturer uses high quality components you can have some leeway, as better "spec'd" components have higher degrees of tolerances. IE the voltage for example can be +/- up to .5 to 1.9 of a volt and they can cope with it..Cheaper components overheat (not discernible on the outside casing) and then fail...usually after the device has been turned off, and then restarted.

You may be lucky, but with something as important as your gig charts reader, personally, I would not be taking ANY chances at all!!

Although as you say you have ordered an iPad ( GREAT decision ) so it is probably academic anyway

Dennis