How do you make a very good thing even better? If it's a smartphone you add a few more cores while cranking clock speed, increase battery capacity and cram in 64 gigs of internal storage. That's the HTC One X+, available exclusively on AT&T in the US. It's the mid-cycle refresh of the HTC One X to keep the phone competitive with the Samsung Galaxy S III, LG Optimus G and other high end phones.

The One X+ has the same design and unibody polycarbonate casing as the One X, but it's available only in gray (the One X was also available in white). For a 4.7" smartphone, it's surprisingly compact and the curves fit perfectly in the hand. The non-gloss surface is easy to grip and polycarbonate is durable and doesn't interfere with wireless radios. Like the One X, the rear camera lens protrudes from the back like a small porthole, making it a little wobbly when you use it on a desk.

If you've read our review of the HTC One X, you know it was one of our top picks in May 2012. The iconic unibody polycarbonate design, superb 4.7", 1280 x 720 Super LCD2 display with 312 ppi pixel density and smooth and reliable performance made it stand out. The display is still one of the best we've seen on a smartphone, and images and text look painted on. Colors are rich and accurate and auto-brightness works perfectly, unlike our Galaxy S III that keeps us in the dark on auto-bright.

The One X+ moves up to the new NVidia AP37 1.7GHz quad core Tegra 3 CPU, making it faster than both our US One X with the dual core Snapdragon and the international 1.5GHz Tegra 3 One X. The phone benchmarks are at the top of the list in all tests and even more important, it feels fast in actual use. Android 4.1 Jelly Bean with its "Project Butter" speed optimizations adds to that perceived quickness and the phone never lags. HTC Sense 4+ skins Android, and we have no complaints. Sense is never overbearing and it brings good usability tweaks while still letting features like Google Now shine. Though there's no microSD card slot for expansion, we can forgive HTC since the phone has a plush 64 gigs of internal storage at a much lower price than the 64 gig iPhone 5.

Just as with the One X, voice quality for both incoming and outgoing voice is very good on the One X+. Call recipients couldn't tell we were on a cell phone and the phone did a good job of rejecting ambient noise via the built-in mics. Incoming voice was clear and easy to understand with slightly better than average volume. As a calling device, the HTC gets a thumbs up, with one complaint: Bluetooth audio quality for calls is sub-par. HTC has a fix in the works for this and it should be deployed OTA very soon.

This is a quad band GSM world phone with 3G HSPA+ (AT&T calls this 4G) and 4G LTE. Like most current smartphones, it has the mobile hotspot feature so you can use the phone as a WiFi hotspot for your computer or tablet.

The HTC One X+ has a 2100 mAh Lithium Ion battery that's sealed inside (up from 1800 mAh on the One X) and we've so far had no trouble making it through a full day (7am-11pm) with moderate use that includes everything but gaming. 3D gaming will drain the battery more quickly than anything else, and gaming drain is higher on the One X+ than on dual core Snapdragon phones. The One X+ also gets quite warm on the upper rear and near the earpiece when gaming for 10 minutes or more. The same is true when using the LTE 4G connection heavily, say to download 20 apps in one session. It's by no means hot enough to burn you, but you'll notice the heat. The phone gets even hotter when plugged in and charging while gaming, an unfortunate Catch-22 since gaming drains the battery fast.

The phone has an excellent 8 megapixel camera with a BSI sensor, a fast f/2.2 28mm lens and an LED flash. It can shoot 1080p video at 30 fps and has features like HDR, burst mode and panorama. It's largely unchanged from the One X rear camera, and it's still among top camera phones in the US (Galaxy S III, iPhone 5s and Nokia Lumia 920), though we'd say the Lumia 920 sits at the top for low light work. The front camera is improved from the One X and is 1.6MP.

The HTC One X+ has Beats Audio, and the audio enhancement works with any brand of headphones. Beats has quite a few settings to choose from, ranging from the classic bass-heavy Beats EQ to classical, vocal and warm. That should help for those who like their music less managed by EQ, but who do appreciate some audio enhancement. The rear firing speaker isn't all that loud and is easily muffled when laid face up on a desk, but music and video sound good through headphones.

The phone can handle HDMI out using an optional MHL adapter that plugs into the micro USB port. We tested this with a few TVs and our Sony AV receiver and it worked fine.

HTC includes their own music app that integrates TuneIn Radio, MOG Music and SoundHound for track info, artist info and lyrics. You can add other music apps to the player if you like. The Music app automatically got album art for our tracks and we like the rich interface and background playback feature. You can control playback from the lock screen, top taskbar and widget.

For those who prefer Google's apps, Google Play Music is here as is the Google movie store and standard Gallery application for photo viewing and video playback. HTC Watch is here for movie rentals and purchase. We tested 720p and 1080p MPEG4 video playback stored on the device and through MHL to HDMI and videos played smoothly with good audio sync.

The smartphone had Beats audio, dual band WiFi, Bluetooth 4.0, a GPS with GLONASS and NFC. While Bluetooth behaved well on our One X, the One X+ has a bug where Bluetooth audio with many headsets and our car's built-in Bluetooth sounded blown out, overly loud and distorted as if the gain had been set much too high. HTC is aware of the issue and they'll release an update to fix the problem. WiFi on the other hand performed well as did the GPS.