Orange Terror Bass 500w Bass head - in-depth demonstration and review - YouTube
Very helpful video. Plug in the best headphones you have and listen closely.
Keep in mind that the Orange TB 500 is not the most versatile head out there, but the inherent tone may just work for you really well.
The M6 is more versatile here. I was not a big fan of the head mainly because of the low mid response - 200-400hz. I felt the M6 cut out a little bit of that range. It's clear sounding, great low end presence, good mids around 600hz and the upper mids, and VERY LOUD. Try one out ASAP.