mmm somewhere in my dim brain when we changed our head unit in our Presara earlier this year something about standard unit in our Presara being a JBL head unit was said when we were choosing our unit..we went with a sweet Clarion head unit and had Monster install it..all is perfect...as Jacko said try the rev switching or maybe something came loose in harness connections? Hope it's an easy find and fix so you can get back to enjoying the cruising,
KAA