ntx-

have you replaced all your speakers or just the front?

Also, how much power (RMS watts) are you running to your main speakers? How much to your subs?

In that case, one reason you're getting so much bass is because your main speakers are running full range- 20hz-20khz. Typically it is best to run frequencies below 60hz to your subs and frequencies above 60hz to your main speakers. Despite what spec sheets will tell you, most full range speakers can do below 60hz accurately. 

Since your speakers are reproducing all frequencies (and most likely doing a poor job on everything below 60hz) you have 6 speakers doing what only 2 should be doing.

I hope this helps you out.

A crossover will help. The only problem with that as the solution is that you'd have to put it somewhere inbetween the headunit and the speakers. I don't know of a crossover that would be that simple.

A better solution would be to get a 4 channel amp with a built in crossover. As far as I know all amps have built in crossovers.

What kind of headunit do you have? How many pre-outs does it have?

The head is an Alpine TDM-7582. I think it only has one set of pre-outs.

I've owned 3 Alpine amps and they've all worked very well. But, stick with their V-12 line, avoid the V-Power. Even though the V-Power are much better than previous lower end Alpine amps, they aren't as good as the V-12s.

If you don't want Alpine, then take a look at JL Audio, Rockford Fosgate, Zapco. There are some others, as well, but I can't think of them. Phoenix Gold used to make some really nice amps, but I have only negative remarks about their new for 2004 products. The stereo shop I deal with actually dumped the entire PG line.

Ever heard of Koiiler Amps?