Personally, if a keyboard has a slightly lower overall output level than others, I wouldn't fix it by boosting it internally. One always HOPES that the guys making it know their business, and probably set it up so even the heaviest, loudest, most busy and dense style doesn't overload the internal D/A converters, or hit the Maxx too hot. After all, these ARE digital devices. It doesn't matter WHERE you exceed digital 0db, once you do, nothing can fix the issue other than turning down the ENTIRE arranger's styles to keep the intended balance.

Unless the outputs are noisy (one assumes they are not) simply run your mixer's gain a bit hotter to compensate for the 'safer' levels. Modern arrangers have so much extra gain coming from master EQ's and compressors, it's easy to take it too far (I'm not a big fan of the modern trend to hype and compress the sound of everything... it simply makes it flat and undynamic once the initial 'wow!' wears off).