Hi guys, advice would be really helpful

Recently I upraded my car sound system (a Toyota Vios E spec , 2010 model) by buying some of the second hand stuff from a friend who has upraded.

What I have now is ,

HU - Toyota Vios E spec Ori Stock HU
Front Door Speakers : Dominations mids/Tweeter with passive xover
Back Speakers : Vios ori stock Aux speakers
Amp: Audiobank 4 chn amp
Subwoofer: Kicker 12" Subs


My Components and subs are powered by my Amp while the back speakers are still by the HU
Also my music playlist is in .mp3 format with at least 320bit rate

My problem is that the music does not feel 'surround' and I get sharp piercing music that isn't very clear to my ears that is annoying to myself and passengers...

Frankly, I blame the installer as his not able to offer any good advices or get it tuned... and the other shops which I have visited recommended me to change to a Pioneer HU...and also to remove the Back Speakers

Need some advices on what to do , to get a quick fix without burning a big hole in my wallet.
Thanks!

The way i see it... changing HU wont solve it...

gain tuning might help...

and does it hiss as well when you play CDs?

Yes it hisses , but only for certain songs... some are good but some are just horrible and too 'piercing' to the ears to listen to