5U4G's will work providing there is enough clearance for them to fit. I know they will fit in the Stiletto heads. I'm not sure about the ACE combos though. The 5U4 and 5U4G is an older, larger (coke bottle shape) design that will produce less sag than the 5U4GB's that ship with current Mesa products. They are about 3/4" to 1" taller than the 5U4GB.