Supe, sushi's comments in particular may be pertinent. The M80s have accurate bass; it's possible that the JBLs were unnaturally prominent in the upper bass and that you became used to that as being normal and good.

A remote possibility, but it actually happened to someone here about a year ago: since the M80s have two sets of terminals, be sure that the jumper connecting them is tight so that the bass drivers are actually working.

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Enjoy the music, not the equipment.