I think you will be OK with the WM12, based on the assumption that it's a bit louder than my GK MB150E combo. But it is going to depend on the band. There is one big-band in my locale, 100% acoustic, which is a bunch of really powerful players who enjoy playing loud. This band can overwhelm my 100-Watt combo.
But with the exception of this unusual band, the MB150E is typically ample for big-bands. With a bit more power, and probably higher speaker efficiency, the WM12 should be fine.
Keep the GK and the louder of the two speakers in your car just in case
