It's one of the great mysteries on head-fi. Almost every mention of Presonus HP4 has met with a suspiciously predictable indifference. It's very popular elsewhere, but on this site it just doesn't exist. It seems no one here has ever heard of it.
I talked to somebody on gearslutz and he said he used it to drive his HD650 and LCD3 headphones and after comparing it with a lot more expensive amps, he realized he couldn't tell a difference in sound quality and sold the expensive amps, keeping only the HP4. Someday, somebody will talk about it here.
I don't know about that. I've owned the HP4 and there's nothing about it that can compare to higher end amps, even something like Schiit Asgard 2 which is not really high end at all. HP4 first of all has very limited power output, then it has a high output impedance, it suffers from channel imbalances due to having cheap volume pots, it hisses on higher volume levels, it has a typical sort of harsh and dynamically flat sound that you'd expect from cheap audio gear. It's not horrible, but you're getting what you pay for, you're getting four 30 dollar headphone amp circuits in one box. Anyone who can't hear a difference between it and more high end amps using HD650's or LCD3's clearly doesn't have the ears required for this hobby or is using some really poor quality source components or badly recorded music. Which is not that uncommon, not everyone who buys expensive audio gear knows what they're supposed to be paying attention to while listening to gear, and people usually expect huge differences, while the differences are actually always very subtle, but that subtle change in sound can be the difference between something sounding totally realistic and something sounding a little off, and if you're really into hi-fi, that little difference means A LOT to you, if you're not really into it, you might not care at all about it. As I said before, K7xx's (any model) are very capable headphones, MUCH more capable than most people think and than most people who own them ever realize, they're one of the hardest headphones out there to elevate to world class performance and they scale like crazy. I think of my K701 as a baby HD800 (baby actually meaning it potentially has 90% of HD800's technical ability, while most headphones in the <300 dollar range couldn't even crack the 50% barrier), because it can sound boring and flat with a lot of entry level gear, but can be absolutely magical with other gear, and even beat the many times more expensive T1's in many aspects. Now it just depends on what someone needs, if you want just ok sound, anything with a headphone jack that has enough power to get them loud enough will do it for you. If you listen to music more deeply and care about all the subtleties, then you'll want something more than an entry level amp.
You see, this is the paradox of today's headphones. 6-7 years ago, headphones like HD650's or K701's were the best on the market (minus a few of the exotic models like Stax that were many, many times more expensive), and people actually considered them to be top of the line, best of the best, best the headphone world had to offer to you, and therefore, people treated them with such respect and used them with high end equipment in order to extract the maximum performance out of them. However, once the headphone prices went ape **** and started hitting those 1000,2000, or 3000's in price, a lot of people forgot that those K701's and HD650's were very good headphones, they became mid range headphones, even though there's nothing mid range about them. People started looking at them as mid-fi and forgot that they require high end gear in order to show their full potential. They're relatively cheap nowadays, but they're not like other 300-400 dollar headphones that came out after them, most of which were designed to work with portable gear and don't really scale up with high end gear because they don't have the technical ability to do so. They're relatively cheap, but that doesn't mean they'll sound very good if you plug them into bad equipment, that's my point I guess.
