70w RMS Speakers powered via Head Unit?

Simple enough question... Can I moderately effectively power some MB Quart Premium speakers that are rated at 70w RMS via a head unit (that typically deliver ~20wRMS)? I know ideally you would want to use a separate amp but it simply is not feasible (due in large part to size constraints) in this install in a... show more Simple enough question... Can I moderately effectively power some MB Quart Premium speakers that are rated at 70w RMS via a head unit (that typically deliver ~20wRMS)? I know ideally you would want to use a separate amp but it simply is not feasible (due in large part to size constraints) in this install in a Miata.

I know pumping less watts to the speakers can be dangerous if the gain on the amp gets turned too high (clipping), but how viable would this be overall. I am not looking for max db or even quality but I am simply trying to figure out a decent solution to the absolutely horrid Bose system that was in the car initially.

Please don't reply with get an amp. I have a very nice Eclipse amp that was initially paired with these speakers, but it and most others are simply too big/heavy for this application. I know there are smaller amps, but most smaller ones are still too large for the application.

So... How bad will it sound if I use these 70wRMS speakers off a 20wRMS HU?
Update: Can't add an amp in the trunk as it would have to go on top of the spare tire, and would take up a bulk of the trunk space. COULD one fit? Yes. Am I willing to make that sacrifice? No. Even if it means I must stick with the stock unit till the end of time. Car is small enough as it is and space is a... show more Can't add an amp in the trunk as it would have to go on top of the spare tire, and would take up a bulk of the trunk space. COULD one fit? Yes. Am I willing to make that sacrifice? No. Even if it means I must stick with the stock unit till the end of time. Car is small enough as it is and space is a premium.

Yes, the sensitivity is 87db. The woofer is mounted in the base of the door sill, and the tweeter is located at the top of the door sill in the stock locations.

I don't listen to music super loud even with the top down. I just like having a moderate volume. I typically never go above roughly ~50% volume at absolute maximum that I listen to from the stock head unit. Given the stock Bose unit IS amped (but is proprietary Bose unit and the voltages are not compatible with other HU's.) Not to mention it sounds awful and there is no definition to the audio. Just wanting a bit more clarity and definition, not more volume.
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