Flat?

Yes, you are just retarded. :lol:
Now, back to business..

Remember that crossovers don't just cut-off sound, like a wall... but rather they attenuate sound, with a certain rate. Most Xovers are 12dB/octave.
Low-pass Xovers "allow the lows to pass" - attenuating off the highs.
High-pass Xovers "allow the highs to pass" - attenuating the lows.

So, picture this:
Low-pass Xover set to 100Hz.
One octave higher, 200Hz, it will be only 12dB quieter.
One octave higher, 400Hz, it will be only 24dB quieter.
One octave higher, 800Hz, it will be only 36dB quieter.
One octave higher, 1600Hz, it will only be 48dB quieter.

And the high-pass is the opposite...
One octave lower, 50Hz, it will only be 12dB quieter...
One octave lower, 25Hz, it will only be 24dB quieter...

Especially with your comment that the sub sounded "quieter when the HPF is enabled" ...quieter, but not completely "off"...
has me thinking...
Possibly, you have the HPF set going to the subwoofer amp.

See, this is one reason I prefer to use built-in Xovers in amps..
There's no question what signal is going to what.

What you WANT is the HPF enabled on your front-speaker amp, I'd set it somewhere in the 80-100Hz range.
And you WANT the LPF enabled on your subwoofer amp, set probably around 80-100Hz also (particularly if you've been happy with it completely missing, you'd probably be happy even with it set to 100! )

I'm confused, anyway, where this Xover is...
Are you using one of those stupid HU crossovers? h34r: [/b][/quote]
Yes Im using my Alpines crossover

And my fronts are also powered by the HU I dont have a seperate amp for them