The KEF can play louder, cleaner bass than the C6B and has no potentially problematic floor/ceiling bounce response issues.
In most other aspects the two perform shockingly similar.
You can compare the full suite of measurements here:
AsciLab C6B Review
www.erinsaudiocorner.com
KEF Concerto Q META
www.erinsaudiocorner.com
And everything frequency response related you can overlay directly here:
www.spinorama.org