The first thing to check to see if the Mic is an issue would be to see if the mic works correctly (without rumble) when plugged directly into the StudioLive 24Ai.
We did some research and found out that the D6 has a low end bump.
If its double preamped, it would have a ton of bass. We suggest the following:
1. swap mics
2. plug mic in direct
3. make sure that RC500 is plugged into the line and not the mic input
If the problem still persists, then contact support by filing a support request with us at http://my.presonus.com.