Using the crossover that came with the speakers is good, no worries there. Using the first two channels for them is where you need to be. As for channels three and four that would be the one for the subs, and you could bridge it as well on those channels, and if you have the bass boost knob that is also from what I can tell for the sub channels three and four.
So, on channel 1 & 2 you put the crossover on full range and then adjust the freq according to what the speakers can manage. Look at the manual for the Dxi's to get a better idea on that. Putting the crossover on full range is fine because, the crossover that came with the Dxi's will divert the signal accordingly. Do not turn on the bass boost on these channels... It will make them sound more bassy, and muddy. Use the HU bass, trebel, and mid for that stuff.
Channels 3 & 4 is for the sub. I think you can bridge that to get more power, but I could be wrong. I sure I am right, but a little hazy on that.
Arun please help with this. Thanks.
At any rate if bridging the last two channels is good. Then plug in the bass boost control to the hole, then set your croosover to lass pass, and then adjust the freq. Now with this, anything that is higher then lets say... 40Hz should not pass through. In fact I would leave that at 40Hz, and call it even. Do not use the bass boost switch on this either. That is what the knob is for. This should be by the driver side of the car.
The volume issue is not at all bad. Basically what they do (Best Buy) is turn up your volume on the HU to roughly 80% max volume. They then adjust the gain, and they try to get this match what the HU is pushing out. They adjust it to where they hear audiable distrotion, then turn it down a bit. It keeps you from blowing your speakers. NOW THE GAIN KNOB ON THE AMP IS NOT VOLUME CONTROL. It is only used to match the signal that comes from the HU.
The over all idea is blend the sub with the front speakers so that they sound as one. I have a feeling that I may be off on some of things by a little bit. I have had an EXTREMELY wonderful weekend, and I am still thinking of this girl from the club so, yyyeeeaahhh my mind is kind of all over. LOL.