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I don't see many many BP builds for home systems. I have been very impressed with the low end on my Iron Driver entry from a few years ago. Outside of group delay issues are there any other negatives related to BP enclosures I'm missing.
The reason I ask is that I gifted some friends a pair of my Crystals and they need a sub. Just for fun I modeled the MCM 55-5670 and came up with this. Getting down into the 20's at almost 100db with an 8" woofer and a small plate amp seems like a good deal to me. The box is a little big but the Crystals need speaker stands.