I am not aware of anyway to change crossovers either.  You’re probably best talking with Sonos about this, but I suspect that it’s a low priority for the majority of Sonos customers, and thus not highly on the list for Sonos.  As well, Sonos wants to keep the system as simple and easy to use for consumers as possible, with trueplay doing the tuning rather than manual tuning.  Sure, they could bury the setting so those who don’t care don’t bother with it, but again, probably not a high priority.

Since you are using a SONOS SUB, the likely assumption is that the SONOS system will select the crossover frequency offering the most uniform overall system frequency response. You have some adjustment capability with the bass control, but it will modify a wider frequency range than a crossover frequency control would work with. You should also experiment with Loudness ON/OFF.

Have to say, I don’t like this “best answer” when they don’t answer the question.

Yes of course the system tries to sett the crossover, but obviously it doesn’t do a good job in this room. Strange to not have the possibility to set it our self. Trueplay normally does a decent job, but for now there is an software error, so trueplay isn’t a option at the moment.

more about the trueplay error her: