stereo car radio to mono speaker setup

Here's my experience then :

Pug 306 DT on stock speakers and a Blaupunkt Bronx MP75 HU : Pretty good
Pug 306 GTi-6 on Pioneer speakers and the same HU above : curse word.; it wasn't terrible but the stock stuff was better. With the stock stuff the bass was there, less volume was required which led to less issues, which I will come to in another car. I remember lending that car to a mate and him saying exactly the same thing; I couldn't argue as I knew it was true. With Pioneer Coxaxials, a fart would give more volume than the HU at full tilt. Thankfully, I ended up going all out on that car, as well, you may was well go big or go home (I'm referring to Rainbow Pro Vanadium Crossline Components, Denon HU, Phase Linear Aliante Si sub, Alto Pro UCS processor & a stack of Genesis Series 3 amps to power the lot). Not a poor list of equipment.

Pug 205 GTi on stock speakers and same HU above (Bronx MP75) ; very good for what it was ; I was surprised; it made those European trips easier to deal with.

Mercedes W124 250D on JBL 4.5" Coaxials : curse word. I was worried I would blow my Alpine 9887R to bits (those to know will appreciate why I was worried; they're not an easy or cheap HU to come by these days, considering what it can do (active crossovers which can do components without the crossover boxes, time alignment, EQ, and in this case, Bluetooth and an iPod connected). The HU would dim and occasionally cut out at high volumes (according to the HU), and I was cranking up the volume very high to get decent listening level. I lived with it as ultimately, I spent more on that car than I wanted to. Yes I did check the earths before you say that and yes, I checked the power supply. The truth was most aftermarket speakers are too innefficent for HUs and required standalone amplifier power. No matter how you look at it, that's a cost you have to add into things. Yes, the speakers are cheaper, but are they after invensting in amplifiers, wiring, ballache of wiring it in etc.
Mercedes W124 E320 on unblown decent stock speakers: Night and day different=ce. Half of the volume level required to get the same output, and a nicer sound. No bright ear bleedy treble/mid-range, and no cutting out/dimming HU.

And yes, I've tried a few things over the years with audio solutions. My experience generally is to either stay stock (ish) or go all out to get decent results. These days, I'm lazy.

When I say blown I mean:
-surround has deterioated
-speaker suspension has gone too soft (generally down to playing more bass than the speaker can handle ; a sub and a high pass filter on the main speakers make a world of difference here)
-Speaker is binding/doesn't work.

To most people, I'm aware blown means not working at all.

In a nutshell, the biggest issue with aftermarket stuff is that the sensitivity of the speakers is far too low to be utilised by HU power alone. Yes you can buy amps, but that addeds complexity and to most people, takes away from originality/"periodness". Some manufacturers do HU power biased stuff but it's not that many at all. After all, everyone needs a cut from any business...