When I turn my MK5 on, I turn the left channel on first and let it the tubes light up. At this point the meter is not fully lit but there is a little back light visible.  When I then turn the right channel on, the meter lights up all the way. If I turn on the right channel first, the same thing happens. The meter light up a little bit but then fully lights up when both channels are on.

If yours is different, then that may mean yours is correct on mine is not...or vise versa. I know that I don't have any concerns with how mine is operating at all but knowing which way it should be is worth looking into.