My Sixxbird's pups sound very different than my Thunderbird IV. The output is louder, smoother, and a bit darker. The IV didn't have anywhere near the output, and the mids were more pronounced and aggressive in comparison. I bought a set of black TB+ pups from Dennis a while back that looked like they may have came from a Blackbird, and loaded them into my IV. They were a closer match, but still not quite exact. Nice pups nonetheless.