Dead Nearfield Monitor?

M-Audio BX5a (I know, QC, little late now.) Bought in Oct, never hooked them up until a few weeks ago.

One was cutting in and out intermittently, now it doesn't do that, BUT it doesn't really make any sound unless I wait a couple hours (to let it warm up??)

Is this a power supply, amp, or speaker problem, and is there a way to remedy this?

Thanks in advance!!