I would think bass extension is possibly in the mid 30s. Agreed 20 is probably not possible in that cabinet.
There’s quite a bit of bass in my 6in 820.2 bookshelf speakers though. It has a nice cabinet. I still use a little sealed 12in sub because I listen to a lot of bass heavy music. My Yamaha A770 is set at 80hz crossover to my Rythmik L12.
My master bedroom is about 1400 cu ft. Most people wouldn’t even need a sub in a bedroom with these speakers but I’m picky.