Question re Subwoofer Connections, High-Pass, Low-Pass

My sub (a Velodyne F-1800RII), includes line-level inputs and outputs 
which permit high-frequency signals to be filtered out and sent through 
the line-level outputs to the power amp, essentially removing the bass 
from the signal fed to the power amp and providing bi-amplification. 
There is a high-pass filter switch which can be set to either 80Hz or 
100Hz (6 dB/octave). There is also a variable frequency low-pass filter 
which can be adjusted between 40Hz and 120Hz (12 dB/octave initial, 
48dB/octave ultimate).

My question is: Assuming the low-pass crossover is set to 40Hz, and the 
high-pass filter is set to 80Hz, what happens to the frequencies between 
40Hz and 80Hz (Ignoring signals that are outside the roll-off curves)? 
Are they simply lost?  In this system, the mains are Maggie 3.6 planars, 
with response extending somewhat below 40Hz, and I would prefer to feed 
bass above that frequency to the Maggies rather than have them 
reproduced by the subwoofer.  But with the above system, it seems that 
they would not receive any substantial bass below 80Hz. Also, what 
happens to signals sent to the mains when the Velodyne low-pass 
crossover is adjusted, e.g., adjusted between 40Hz and 80Hz or above?

Presently I am not using the high-pass connections, and instead feed the 
full audio frequency spectrum directly to both the sub and the power amp 
(the preamp output is connected directly to the power amp and to the 
sub). This is because I had concluded that signals fed directly from the 
preamp to the power amp, without any processing or filtering, would be 
cleaner and provide a more transparent sound from the Maggies. On the 
other hand, it might be beneficial to remove the very low frequencies 
from the amp and the Maggies for greater efficiency.

Any advice or suggestions? Do I need an external electronic crossover to 
resolve this issue?

Jim