Is there a limit to how long of a cable you can send phantom power? I just bought a couple of AT lavalier mics that can be phantom powered with 25' cables and connected to a GL2 with a Beachtek DXA-6. When I turn the phantom power switch to ON, the audio is quiet and terrible with lots of noise. BUT, when I turn the switch off and use a AA in the lav mic, the audio quality is perfect.

I'm pretty sure the battery is fine... the power light comes on. The only thing I could think of is that maybe the cord was too long to send the power through.

That generally indicates the cables have a bad connection or a break somewhere in the 2 conductors that carry both the signal and the phantom power. One half of the signal can still get through the cable if there is a break in one of these two conductors so the mic may work on internal power, but phantom must be carried perfectly or it won't be "phantom" anymore and will wipe out the audio with noises and trash.
With good cable you can go many hundreds of feet with phantom power and mic signals without a problem.
Which AT lavs did you get? Do you have a way to test the connections of the cables?

It got the 803b model. Let me try the other cable and see if that one works any better.

Yup, the other cable worked fine, so you nailed it. It's just defective. Now to call and exchange it.