From what I heard straight from a Mesa tech (not Mike B.) is that the later DC5 has a bit more clean headroom than the first generation (which has the LEDs on the faceplate). That was the gist of the difference, other than minor stuff like the EQ switch and footswitch changes.
The simplest way to tell is the first gen DC5 has an LED for clean/lead channels to show which has been selected. The "B" version has no LEDs and a painted white rectangle around the GEQ toggle switch.
FWIW, there is no "lack" of clean headroom in the "A" version that I can discern ...like other Boogs, this is one *loud* amp for it's stated 50 watts, and she can stay clean and spanky at serious, real-world venue volume. But having never played a "B" I can't attest to the "B" being better or worse in any respect. Hope this helps.