When I first put an amp in my car with concert sound II in '88 I used a two channel line level converter in the trunk on a Crutchfield (PPI) 20 watt (RMS) x2 amp and pile driver 4x10's with the fronts and doors ran off the stock stereo.
I felt the 20 watt amp in the rear made all the speakers the same level at the drivers seat (gain knob at 60-70% and stereo fader in the middle) with enough punch to fill out the lower end.
I would say it was similar to a 6.5- 8 in sub. Not loud, just nice.
In comparison, I had some 6x9 Infinity reference speakers in the rear of my '90 Regal with a Crutchfield 50 watt RMS x 2 (gain knob at 60%) using the stock stereo with a converter that people thought I had some 8" subs in the trunk.
I have one of these four channel converters in my '94 Cutlass Supreme Convertible hooked to a 40 x4 (RMS) at 60 % gain for the dash and door speakers and a 50 x 2 (RMS) at 50% gain for the rear side speakers with good success.
I'd have to look to see the gain setting on the converter, but I think it's around 50%.
It's got (4) gain controls, (1) for each channel and built in ground loop noise isolation. You could connect it pre-amp and run it to a 3.5mm aux input, although the volume would be controlled solely off the device.
https://www.ebay.com/itm/AAMP-OF-AM...ERTER-Black-/191950519919?hash=item2cb1247e6f
The audio control lc21 looks like a nice unit, probably better quality than mine.
Try to get an amp with no more than .05% THD for good sound. More than that, you'll get some background hiss. Less than that you won't notice a difference over your exhaust and turbo whistle.
One thing to remember is the the stock stereos start cutting bass at 75% or so.
If you set max volume at 50- 60% volume you'll get much better sound.