Are they that bad though? Keywords incoming, share and share alike etc. Maybe they don't provide much in terms of say... a psymusic discount, but they do provide regular content, which is something I can't purport to do.
Maybe we should stipulate they provide one conspiracy theory per sales post, e.g. psytrance is corona-antidote, and spread some rumours that every freeparty in London has fewer reported cases than are prevalent in the entire north of England.
Maybe there should be an annoying marketing wank section of the forum and they can post all their shit there and leave the music production forum free for people to ask and answer production questions. Seems like a more tidy option. To be honest it's put me off reading or posting much when the sub forums look like a ghost town populated by one person posting marketing material to them selves.
As for how low sound systems go. They go as low as they're set up to in any particular venue. Outdoors I'd imagine they can go much lower as they'd be no need to accommodate the acoustics of an unclosed space. A big enough speaker with a powerful enough amp should happily be able to produce Infrasounds. At a guess anyway