Sarah B

It sounds like you have the stock audio system. Really easy to tell if you have the JBL as it's embossed on the front speakers. This is a good improvement over the useless stock system, though it's still not that good. I got rid of the complete stock system when my car was a month old and had  pro install. The rear speakers are totally useless and not worth bothering with. If you want a reasonable upgrade fit the Focal replacements to the front. The head unit is not good either and doesn't have an output for an amp. All depends how far you want to go of course. 

Adding DSP to OEM head unit solves quality issue along with good speakers, however it does not solve power as OEM unit is not more than 20 watts. Amp will do the task, but amp + dsp comes expensive and adds weight as well. Aftermarket unit is thousands times better in power, quality and features. 

Sarah B: check what means "upgraded one". JBL? Some custom speaker replacement, eg. to those focals? Headunit replaced? Anything else? Or T&G2 in place of T&G of first generation .. T&G2 IIRC came .. hmm, don't remember exactly .. maybe since MY2015, or maybe since restyle MY2017 ones?
If my '99 corolla with stock stereo door speakers with only change being alpine mp3 headunit in place of stock tape player sounded MANY times better then stock twin audio multispeaker setup designed/made 15 years later .. that says something, lol :D. Luckily car makes up for it with much more driving fun.

You may look at my topic for custom audio system as well. 

It's not just headunit that sucks on twins, or rather it's not THE worst part from all that make up result of badly sounding audio.
Bad sounding speakers, lack of sound insulation, lack of amp or lack of own power from standalone stock headunit.
There are several options to make sound better, and changing headunit might be not the main priority. For basic budget audio enhancement i'd install focal 165toy speakers in doors and switch off sucky rear speakers. Step up - add also some sound insulation (though this can well fit in "base" package). Another step up, add also amp and sub. Another, replace all the other speakers too. If not the amp, then headunit to aftermarket one from leading vendors like sony/alpine/pioneer. Another step up, good well assembled set tailored to our cars (eg. OEM+ one, of amp, dsp wiith soundprofile tailored to headunit used (including stock), speakers upgrade, sub in boot). Another step up, good and expensive upgrade of anything at money-no-object budget at some big well known car audio tuning shop.
From all these i'd probably consider 165toy focals+some insulation, and OEM+ offering (it's oem+ audio US firm made set, not toyota optional JBL audio upgrade, which costs as much but sounds worse), or rather just first, as all "free money" goes with higher priority to track use wearables/performance & suspension mods with sucky audio lingering at low priority "nice to fix" but "i don't care enough" :).

The standard speakers weigh about as much as a pub coaster and are just as soggy. They're clearly the cheapest they could possibly be.
The only factory upgraded sound system from Toyota was the JBL kit, which costs a packet, and shouldn't offend you even if it's not the best value for money. You can confirm this by finding the JBL sub under the boot carpet.
The head unit is tolerable, and it's tricky to find ones that fit flush with the Toyota fascia, so that's a last thing I would attend to. I aim to replace it eventually, but more for Android Auto support than anything else.

Seconded what Church has said. The Focal 165toy speakers are a simple upgrade and a massive improvement over stock. For ~£115 you can't argue!
I upgraded the headunit to a Pioneer one (the Subaru base unit is shockingly bad) and added a tiny bit of sound insulation and the whole thing is night and day difference.
Not sure I'd bother with going any further i.e. amps and subs as the car is a flimsy thing.