I had the oppertunity to listen to the 904, Jub, and the 415 bins in the same day. All with K402 atop.
Just IMHO the DIY twin 15" driver Cornscala DBB bass bin is a much better sounding LF unit than the 904. I can't say why,but only what my ear told me. The cabinet volume is the same. Could be a difference in port size or the drivers wired in parallel vs series. Just don't know. ?????
The sweet Jub LF is just beautiful and is a given.
That said, the 415 bin is a horse of a different color. It forms a very nice bubble of sound with the K402. The best mate with K402 that I have listened to so far. (other than the Jub bin) The 415 has a quility that none of the others exhibit. It has the LF reflex "slam factor". In your face, a boot in the chest. Minimal woofer excursions. 107db efficient as I recall.
Only my observations and for what they are worth..Hope they help.
tc