Gravely? Hmm, time to check how tight your mounting bolts are, they may be too tight. Instructions below.
http://www.scumbackspeakers.com/mount.html
How to install your speakers for maximum tonal enjoyment.
When mounting your speakers, use caution. Vintage style frames (thinner) require that you mount the speakers using an X pattern method.
A) Tighten all bolts/screws finger tight till the bolt meets back of the speaker frame.
B) Hand tighten in a cross frame or X pattern like a head gasket on a car engine. DO NOT TIGHTEN in a clock-wise circular fashion as you can warp the frame.
C) Tighten 1/4 turn on each bolt in an X pattern in one pass, then the other 1/4 to 1/2 successive turns in successive passes. This will compress the gasket enough to ensure a proper seal, and without warping the frame. I recommend no more than 1 full turn after the bolt meets the back of the frame, usually 2/3 to 3/4 is good.
Failure to do this could result in a damaged cone, bent frame, or unusually high shrill, or muddy bass noises from over-tightening.
DO NOT USE AN ELECTRIC DRILL SET TO A TORQUE SETTING!
The J75-LD is a cross between the M & H type speakers with a large dust cap. So it has a lot of the M75 mids, but more bass and upper mids due to the heavier magnet. It won't have the high end the heavy magnet V30, and since the V30 has a small dust cap, trying to EQ the sound between the large dust cap and small dust cap will be tough. That's because the large dust cap rounds off the high end slightly, while the small dust cap on the V30 doesn't.
I never recommend mixing small dust cap and large dust cap speakers due to this. But hey, I've been wrong before, so try it. I'd put the J75-LD on top, V30's on the bottom.
Good luck!
Jim