Well what are the tweeters?
Do NOT bridge the amp if you're tweets are sounding bad...... first thing i would do, is look at how you installed the crossovers.
Did you do it the proper way, and put the crossovers next to the amp, and run 2 seperate pairs of speaker wire per side to the front.... or did you run up 2 pair, and then connected to the xover under the dash..
if you did the latter, chances are you had a lower quality set of components, and you are/were picking up massive engine noise on the components, and it may have fryed the tweeters.
If you want to try something, i'd say turn you're car to the accessory mode, turn off subs (they would drain the battery too fast! lol) and turn on you're components, turn up the volume, and then mess around with the gain settings on the amplifier, there's also a chance you're amp is picking up noise before amplifiying the sound, therefore when the car is on, and the gains are set high enough, it will amplify noticably the engine noise....
But yeah, tell us what kind of components you got, so we can see if they really are 90w rms or pyramid 90w rms (theres a huge differance! lol)