Boosty
Let me get this right...
all your speakers sound trebely? or just the fronts.....?
I have Infinity Kappas 3.5" in the front of my Regal and Infinity Kappas 6x9's in the rear with adapter kit.
are yours the Kappa's or the Reference's?
Mine used to be in my GN ( God rest her soul) and I worked at and installed car audo for 2 years , my GN with that set up plus Door pods (5.25" Kappas) &( no amps) rocked!!I sold a lot of installs alone from just my GN.
They are now in my 455 cuin "82" Regal.
The point I getting at, is one you can cap of the the treble to the fronts and/or try adding some type of foam unsder the front speakers, to create an enclosure (Lot less reflection) ... as for the rears make sure they are installed right with the conversion kit...Mine alone could hit some low bass notes( trunk was air tight acted as their enclosure), from "Whom the Bass Tolls" great CD to use to sell installs, and the Recent "Dirty Vegas"..... all with a Pioneer Premier 35wX4.
Check all those first.....also check that those fronts are snugged down good so no air escapes.
WIth my 3.5" Kappas they had to be mounted from under neath with a spacer ring I made, so they would "bulge" up the speaker dash cover.I also had to cap them off because to much bass.
It sounds like youve covered all the bases with your install, even with stock 4x10's it should give some bass.
With your 6X9's you actually have mids and Highs plus your bass cone,those mids and high might also contribute to you hearing extra treble, (those Infinity's also are very "CLEAN & DISTORTION free speakers) so you may hear stuff youv'e not heard before.
With that said what Im about to say may get you upset......
You didn't need that additional mid and high on the 6x9's , just the mid's...
You see your ears face foward, they pick up the higher decibles and are the closests to the front dash, they pick up everything.
Except for bass, wich is behind you, your ears pick up the Bass wave instead, reflecting back off the windsheild , so there's no need for the "highs" on the rears cause you actually dont hear them , but they do contribute noise, wich may sound high or trebely or sound in a form of distortion.
Thats why the factory 4x10,s as limited as ther are carryed a good sound along with rest of the system to contribute to a very user friendly sound in the car.
I hope this helps
let me know what cha find
later
) 
