That's weird, because LMII without filters and MB500 set flat (mids to 1'o'clock and bass and treble to 11'o'clock) sound same to me on the same cab. I do not think D class module has a certain sound, it is the preamp that "colors".
But cab does make big difference, so it depends what cabs you used when you tried GK. Markbass cabs favoure high mids, and GK cabs are flatter, generaly speaking of course.
True, MB500 is discontinued but can be found used in EU around 300€.
Anyways, it was just an idea, good luck in your search