Following my previous post I tested Logic Pro X (10.0.3) with Lexicon I-ONYX 42s with new drivers 3.00b14 and the whole system works fine. So everybody download the new drivers from http://www.lexiconpro.com/en-US/products/i-o-42
iMac 3.06 Core 2 Duo, 2GB 1067 Mhz DDR3, OSX Mavericks 10.9.2, Lexicon I-ONYX 42S, Logic Pro X 10.0.3