Weird Stereo Issue - Need Help Please

First off, I had an issue with one of my Bullet Hollow Point tower speakers and Jaime went way out of his way to take care of me!! Thanks Jaime!

But now with the new speaker installed (I have 4 tower speakers) I am getting a really bad buzzing in the OPPOSITE 2 speakers, the drivers are vibrating pretty hard??

I pulled one of the speaker leads off the new speaker and it still did it, then I pulled both the RCA's jacks out of the tower amplifier and it still did it??

The in boats are fine??

Other than the bad speaker, everything was fine before I pulled that one speaker off??

Any trouble shooting help would be greatly appreciated!!

Oh, and I should mention it does this with the boat not running.

You have to fully isolate the issue. You still have the amplifier, speaker terminals, and tower wiring to consider.
Re-check the voltage at the amplifier terminals plus the ground. Probably not the cause but it must be eliminated.
Run a temporary jumper wire from the amplifier output in question to the speaker thereby circumventing the tower harness. No input RCA. Try the same with a different speaker.
If the noise is really horrific, place a 100 mfd non-polarized capacitor in series with the test speaker to offer some protection while going through your tests.
From your description thus far, I suspect that you have an internal amplifier failure.

Sounds like something might be potentially grounded somewhere.