Is the ground hooked up properly?
Measure resistance at the plug:
1: Tip to sleeve
2: Ring to sleeve
3: Ring to tip
1&2 should be the same within a few percent (perhaps 10%)
3 should be equal to 1+2 or put another way 1&2 should each be half of 3.
Mounting the drivers slightly behind the baffle should not have a significant effect on the sound, provided the driver membrane does not touch the actual baffle. In the pictures you posted it looks like there is a few mm clearance so this should not be an issue.
I haven't tried this test yet....I am going to have to try this to see what I get. thanks for the tip.