Crossovering the 208's at 80 Hz is almost a criminal offense. They don't need a sub. My marriage to subwoofers dissolved into an acrimonious divorce.
I think that several people already have said my opinion above, but I still like to make a comment myself.
In my system I have two pre-amplifiers, one analog with HT-bypass (Advantage P1) and one digital surround processor (Meridian G68ADV+HD621). With the first I have only my two channel setup with F208 without EQ, with the latter with sub and EQ. This means that I can compare easily with just turning HT-bypass on or off.
I can agree that the F208's can produce bass enough for most types of music, but the extension of a good sub adds an extra dimension to the music. When we come to movies with really deep bass there is no comparison.
I don't know if you have read the post I linked to, but in that post I explain that we tried crossover frequencies 40-150 Hz before we found 80 Hz to be the best in my room. As the room have a great influence on the sound the electric crossover frequency is not always the same as the acoustic crossover frequency. In my room there was peaks around 40 and 80 Hz. When we inserted the plugs in the bass reflex ports of the speakers these peaks were lower, but we still had to add som PEQ filters in Meridian G68ADV to even out the tone curve.
The result with sub is that I have a tone curve with F-3 db@5Hz, shown in diagrams in the previous post that I linked to. You will never come close to this with four 8" drivers. In my opinion the integration is very good in my room. I can't hear that the bass are coming from a particular place, it's just there. This is actually one of the things I really like with my F208's, the sound is just there. My previous speakers was Mirage M-1si and F208 was the only ones I've heard that can disappear in the room as Mirage can.
So, even if my F208's can produce great bass, it's nothing compared to what they can do together with 2x18" BMS in an IB configuration!
I'm sorry if it's difficult to follow what I write above, English is not my native language.