Modifications of car amp for better sound

[celestian]

I bought two broken car amps (big enough for possible tweaking) and after repairing and short listening test of the first one I revealed very "grainy, sandy sound, flat and compressed mids, somehow not tight bass and totally messed higher tones.

My aim is to tweak them to make them sound great (or at least more than good) at a reasonable price.

They are both Hifonics Atlantis, 2x175W/4 Ohm, with SMPS at approx. 27kHz with 5+5 IRFZ44 and TL494CN, making rail voltage +/- 46 to 48 Volts with good filtration.
Reverse-engineered schematics (I hope did not miss anything) attached.
These four diodes are zeners at 22 Volts each, Q7 is at heatsing with all 6 TIPs. Total equipment consumption is 700mA at 12 Volts, I expect the bias of TIPs be very small (I measured less than 1 mV at emitter resistors).

Most of you guys are experts in schematics, however I am a little bit missing a correlation between different approaches to overall amp construction and the SOUND. One of ways is total distortion minisation - Halcro, Error Correction, Douglas Self... an I prefer this sound.
One of my question is: How to make (simple?) amp with only(mostly) 2nd and 4th higher harmonics to make it "pleasurable" for ear.

My idea is to put Upupa Epops's (DPA440) driver stage on separate board connecting to last 1-2 stages of the original circuit. Either leave original TIPs or replace by some newer japanese silicon.

Anyway, some specific questions:
- Do you think of any "easy" modification to try before complete make-over ? / the only planned now is 4558->5534 upgrade/
- Can I leave TIP35C/36Cs (good old dinosaurs) and expect any good sound out of them (at www.zero-distortion.com is a hint to use some tricks to make out a good sound of them. Anybody familiar with that) ?
- Is replacing several 4458's Op amps in input crossovers by 5532's sufficient for car amp, or should I head for more expensive Op amps ?
- anybody with experience with SMPS filtration level/ shielding/ EMI and its impact to sound in a car amp ?

Maros

[lumanauw]

Have you tried this question in Car Audio forum?

Hifonics? The man who built/design it is also a member here. MOER, (Stephen Mantz, now with ZED Audio. Why don't you ask him?)

[celestian]

lumanauw:
yes, there are some questions for the CAR forum, however the upgrade of output stage/possible modifications are more related to Solid State amps, therefore I put it here.

Too many questions in one topic though, but I hope somebody will throw some ideas...

Maros

[celestian]

Attached is the schematics that did not appear with the first message