Smartphones that have stereo speakers (2018)

Samsung Galaxy S9 and Galaxy S9+
The latest high-profile phone series to get the stereo speakers. Instead of utilizing two front-firing speakers, the Galaxies utilize the fairly new method (made popular by HTC 10) of pumping out most of the bass and middle frequencies from a bottom-mounted speaker and then use the earpiece as a tweeter that shoots out focused highs.
Sony Xperia XZ2, Xperia XZ2 Compact, Xperia XZ2 Premium
Asus ZenFone 5 and ZenFone 5z
The new midrange and top-tier ZenFones were announced at MWC 2018, just a couple of days after the Samsung Galaxies. They rock the cool new all-screen front design, though, with a controversial notch of its own. Asus talked up the ZenFone 5 quite a lot for its cameras, power, and features, among which we have the dual speakers.
The setup here is also of the bottom speaker for bass / earpiece for treble variety. Unfortunately none of the phones have hit the market at the time of writing this, so we can't really tell you about the sound just yet.
Huawei P20 and P20 Pro
About the speakers — they are of the bottom woofer + earpiece tweeter variety. We found them to be among the loudest smartphone speakers we've heard. And they sound pretty balanced, too, so kudos to Huawei!
Razer Phone
Razer's very first smartphone is unapologetic in a lot of different ways. It came out with a 16:9 display when everyone was racing for that elongated 18:9 ratio, it's large and boxy and stores a duo of front-firing speakers in its enormous top and bottom bezels.
As a result, we have a nicely separated, loud, and meaty stereo sound.
HTC U11 and U11+
HTC has been putting stereo speakers in its flagships since the first HTC One, back in 2013. In 2016, when it released the HTC 10, it switched from the front-firing drivers to BoomSound HiFi, which is basically the woofer + tweeter setup. Each speaker has its own amplifier. The bottom one pumps out a bassier sound, while the phone's earpiece hisses out the higher frequencies.
The HTC U11 and U11+ come with the same BoomSound HiFi system, though thankfully, they also have a volume boost. The U11+ in particular gets almost as loud as the Razer Phone. And the sound quality here is quite good as well — a nice deep bass, a mid-scoop that clears out some of the mud, but doesn't make the phone sound hollow. We were really happy with the improvement over the HTC 10.
Google Pixel 2 and Pixel 2 XL
Google also joined the stereo speaker crowd with its latest flagships. The Pixel 2 and 2 XL both rock front-firing speakers on their top and bottom bezels. They are on the loud side, but they are not the deepest-sounding ones out there. There's also a bit of mud in the mid range, but overall, it's nice to see a loud front-firing setup on a contemporary flagship.
Apple iPhone 8, iPhone 8 Plus, iPhone X, iPhone 7, and iPhone 7 Plus
Apple has been rocking stereo speakers since the iPhone 7. On the 2017 generation of iPhones, we get a slight volume-boost, but a pretty similar sound. Thanks to a bottom-firing woofer and an earpiece used as a tweeter, the iPhones can have a deep and detailed bass (as much as the size permits, that is) and a strong presence up top.
A couple of put-offs — they sound a bit too scooped for a mid-lover's tastes and the earpiece speaker can break up easily at volumes above 80%.