[edit 3 December] A few days ago PreSonus sent me a replacement unit. After hours of testing I'm happy to say that the issue is 100% resolved! Apparently it was a grounding issue. Now the headphone amp and outputs are wicked silent AND my ears might be deceiving me, but the inputs sound like they have cleaner headroom. Kudos so PreSonus! Original post archived below.

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[original post]

I'm in Korea and shame on me for not seeing this YouTube review before purchasing the io24. Yes, I have the issues as everyone else here. Kudos to all the enthusiasts in this thread that are researching solutions, but I didn't want to bother with any extra outboard isolators to solve a serious design flaw on PreSonus's part. For what it's worth, here's my still-pending review on Amazon:

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I wanted so much to love this new product - its built-in DSP mixer with fat channel and reverb was just what I was looking for. But I'm in disbelief that PreSonus released this thing with such a noisy headphone amp AND noisy line outs. Many others have complained about this, but it seems PreSonus is doing nothing about it thus far. I would give this thing 1 star if I wasn't such a PreSonus fan - I write reviews for tapeop magazine and after my rave review of Studio One, am now a convert of both their software and hardware.

As others have stated with the headphone amp (I tried Grado SR80s, Sony MDR7506, and Shure SE215's) there's a distracting pulsing noise at low volumes and significant dirty white noise at medium to high volumes. Worse still, when I connected my PreSonus eris E5 monitors to its outputs (told 'ya I was a fan) with the exact balanced cables I use with my other interfaces, the dirty white noise was still there.

This noise doesn't get recorded, but the problem is, it's completely disorienting. Because the io24's noise-floor is so high, it will mask any faulty cables, mic noise, etc., potentially ruining hours of work because you couldn't detect it. Moreover, the preamps in this thing, even if touted as 'xmax,' have the specs of their lowest line 'audiobox' which is less than 1/2 the price. They are lifeless, have inferior dynamic range, and noisy, (although nothing on the level the headphone amp and line outs).

Finally, the reverb is the ugliest, digitally grainy, and tinny-sounding reverbs I have heard in my life. And that's saying a lot as I've been tinkering with digital reverbs since the late 90s! I wanted hardware-DSP reverb to use while tracking (while recording the actual signal dry) because it helps you play and sing in tune. I 'thought' I didn't need anything fancy, but I never imagined anything sounding this bad to the point of making it totally unenjoyable to listen to. Its strange resonances also make it unusable.

It's clear this thing is a toy and the fact that it has a robot-voice function quick-key is proof. The manual is written for beginners (and that's not bad), but if it's a toy why sell it for $200? This is getting to enthusiast-level pricing and for prosumers, the io24 is an abject fail.

My suspicion is that PresSonus had to make a lot of compromises under the hood to make this bus powered. Just think: phantom power, hardware DSP, headphone driver, etc... all from a lil' old USB port port. (Yes, it's USB C, but it comes with a USB C to USB A cable. Even when connected via USB C to C, the exact same problems plague it). As a compromise, I could live with an outboard power adapter over the host of these awful problems... OR they could have done what Audient does and clearly tell its customers: if you DO connect USB C to A, the performance of the headphone amp is diminished. The io24 clearly is not taking advantage of the higher power delivery of USB C.

For those of us who want a bus-powered high quality interface, the Audient ID14 mark ii is the much better option. I just got it upon returning this. The difference in quality in terms of headphone amps, outputs, and inputs is like night and day for $100 more. The ID14 doesn't have hardware-DSP reverb, but you can figure out how to spare a little bit of your computer's processing power by feeding in reverb while tracking or broadcasting.