Yeah it doesnt really give a satisfactory redult. You havent said what the problem is but ground loop isolators can stop the alternator buzz.
I found the lear amp more forgiving on the buzz! The HK one sounds a bit better.
The trouble is you end up losing a good proportion of the low end, that bit that makes gives music a bit of depth. Both lear and HK amps sound gash with an aftermarket HU. The crossovers in the amp mess with the HU crossovers i found.
The easiest way to make it sound half tidy is to ditch the amp up front and rewire the mids and low range in series, replace the low end for a nice 3 way speaker and wire back to the HU power amp. Therefore ditching the tweeters. If you can replace the mids too then all the better.
There are other options but its all complicated by having so many channels to feed. Re-wiring the fronts only is a bodge, but just easier. You can leave the rears on the amp and just fade them out, so at least rear passengers can hear something.