Here's the official page:
https://www.rme-usa.com/adi-2-4-pro-se.html I got mine on a sale, about 10% off, which I have seen recur from time to time. For me, that discount was enough to cover cables and a carrying case for it.
I love it. Absolutely my favorite amp. It is clean, controlled, and detailed in a way that I heard from nothing else, including a bunch of Topping and THX amps. Unlike them, it also retains musicality, dynamic impact, and soundstage. It doesn't sound sterile or thin. What it tends to do for any headphone is wring the best possible performance from it, which means the best possible detail, texture, layering, instrument separation, and usually soundstage. It can also help reduce things like bass bloat, treble glare, and smearing because of its exceptional control. It can often make bass-light headphones sound not bassier per se, but easier to hear what bass is there. Same with mids/vocals that are pushed back. It's not that there's now more of that sound there, it's just easier to pick out of the mix and focus on. That said, the sound signature of headphones get zero help from the HPA4. They got a weird mids dip? You're definitely going to hear it. Those headphones too trebley? That's not going away. That helps dial in EQ quite well, but the amp isn't super-forgiving. Yet, you pretty much don't need to worry about synergy unless you know the headphone has flaws. It handles high-z and current-hungry headphones with equal quality. Highly sensitive IEMs or low sensitivity headphones (down to 87 db/mW, at least) no problem. Nothing ever makes it sound worse. It doesn't change with cable swaps, SE vs balanced, or audio drivers either. It's completely indifferent to all the little fiddly stuff people do to eke out more audio quality. Frankly, it's kind of boring. Well, until you're actually listening to music. Versus the 2/4, there's only a small bump up in quality of most things, but a few demanding planars, like the LCD-4 and 5, sound much better on the HPA4. To the point where I cannot listen to the LCD-5 without EQ on the 2/4, but I frequently do on the HPA4. And, it's not just a matter of power, the 2/4 can actually output nearly 1W more per channel. But, the HPA4 resolves those heightened mids enough to prevent glare and keep them clear. All that gets increased just a bit more using the Benchmark DAC too. Anyway, huge fan of the HPA4. I hope you get to try one someday. It may not be your preference ultimately, especially since you're asking about it on a tube amp thread, but it's a certainly worth a long listen regardless.
