We all have read that relieving the bass frequencies results in a bit more max SPL, the question though is this true. Can the bookshelf woofer play a 200Hz note louder when it isn’t also playing a 40Hz note? I don’t recall ever seeing anyone test this.
Certainly it is true for actives that automatically compress low bass at higher SPLs, like Neumanns and Genelecs, but it's a good question whether or not it works that way for passives. I have no idea tbh. Sounds like something that'd be worth testing, and not that hard to test...