FWIW, flared vents of the same length, base diameter as an un-flared one will be acoustically shorter, so will tune it a bit higher, sometimes enough to be audible. Also, the polar, roll off patterns of the 511, 811 are different, so the M19's XO may not work well with the 511 plus the bass bin ideally needs to be inverted to get the woofer closer to the horn regardless.

GM