The BL80 is VERY dark and neutral sounding. Lot's of lows, low mids and zero highs. Very tight under pressure too.
The Vet30 is in fact dull in comparison to the Ret30. But that would be desirable if pairing it with the BL80. Since the Ret30 just dominates the BL80 with it's upper mids and stinging treble, cutting right over and through the BL80.
I haven't heard a K100 in person, but my impression is that it has a lot more character and high end bite than the BL80. Just my perception from afar though....
I love my Retro 30's, but I really only like them by themselves.
I've paired them with GT75's, ET65's, BL80's, Vet30's and Greenbacks. I liked them with the GB's best, but they really perform best on their own IMHO.