This MAY come as a bit of a surprise to many of you here, but a really good headphone amp that drives the K601 really well was made by AKG themselves. It only stands to reason that AKG would voice their amp to their headphones. You never hear audio reviewers mention it, but the amp was geared to the professional market, and sold at a price point of low 4 figures without heavy discounting available as usual in the consumer market. Also, the amp was not merely a headphone amp, but a system of components. It is the HEARO 999 Audiosphere II. Besides being a wireless headphone of the highest quality available at the time, it was a transmitter of very high quality that could switch between three different frequencies (you could have three of them in use simultaneously, for remotely checking different sources), it was a very good outboard DAC that beats the DAC's in most consumer gear (I'm using the DAC instead of the one in my Sony CD/DVD player), it was a synthesized 2-channel surround sound processor, it could function as a preamp in an audio/video system as it had one analog and two digital inputs and a variable analog output, and it has that very high quality hardwired headphone amp for driving inefficient professional headphones, of which AKG are some of the hardest to drive. The HEARO 999 wireless headphones had a lowball discount price of $500 in their day, and the whole setup with one headphone, one transmitter/DAC/amp, one recharger, cables, foam lined hard case (designed for hard pro traveling use), extra leather earpads, and a very thorough owner's manual usually discounted no lower than $1100.
I have driven my K601 cans from the output of my B&K Pro-10MC headphone jack (no slouch, it drove my old 600-ohm Sennheiser Pros with no problems, or anything else I plugged into it), my Technics CD player headphone jack, and of course the HEARO 999 headphone jack. Order of sound quality in rising older is Technics CD player, B&K Pro-10MC, and the AKG HEARO 999. The 999 boosts the bass and treble output a bit, most notably the bass, and makes the K601 more neutral. In fact, I would be hard pressed to tell the difference in sound between the K601 and HEARO 999 wirelesss headphones. I would more quickly notice the extra weight of the wireless headphones. This is confirmed by plugging my DT880 cans into the 999 amp and now they are often too heavy in the bass for me. They never sounded that way from the other two sources I listed. In fact, they are almost bass monsters when driven by the 999.
The 999 amp drives 25 feet of Grado headphone cable like it isn't there. The volume control stays below halfway up on the DT880 and K601 most of the time, so it has power to spare and I would wager it could drive any dynamic headphone in existence without breaking a sweat. A pro user of AKG headphones would not tolerate a headphone amp from AKG that changed the house sound of their headphones. From the 999 you will hear that the K601 has that trademark AKG midrange centered sound, but you will not be calling the bass or treble weak. I gave away both my Grado SR200 and SR60 headphones as they were uncomfortable and the sound could wear at you over time, so can't say how the 999 would treat them. My Sennheiser headphones need new earpads, and I will not be getting any as the cords broke about every year and I got tired of replacing them. The AKG and Beyerdynamic cans have been no trouble for the past nearly 4 years of use and they seem of a higher build quality, especially compared to the Grado cans. When I bought the Grado SR200, the first two I examined at the dealer had defective cloth speaker covers peeling away, and the pair I bought had a cracked yoke where the headband stem is pressed in. Within an hour the SR200 felt like a torture chamber. The K601 delivers a lively sound for pop/rock music but is much more comfortable and listening fatigue free than Grado.
If you can find a used HEARO 999 setup on the used market, you will be getting a top amp for AKG headphones and a whole lot more besides, a lot more than just a wireless headphone. The DAC will stand up beside $500 separate units, no sweat. The HEARO 999 was a killer outfit, and the audiophile (audiophool?) press missed this one completely. Of course, that is true of much good gear aimed at the pro market that would fit well into a consumer audiophile system. Most of the HEARO 999 sets were sold to professionals, and they do not suffer from audiophile nervosa and will hang onto their gear for as long as they have a use for it. Good luck with finding you a setup!!! You can't have mine, unless you pay WAAAAYYYYYY more than I paid for it. I consider what I paid as chump change for what I got, maybe the best deal I ever got in audio gear. I learned about the HEARO 999 from a German audio engineer who uses it every day in his mastering studio. In fact, he was responsible for one of its design features as he personally knows the AKG bigwigs and he has used their gear for many years. If it's good for a high end mastering studio, it MAY be good enough for you, but with audiophile nervosa and many audiophiles more interested in what others think than letting the gear tell them, they will read what paid hucksters (pro reviewers) have to say rather than those who pay their own money to use it. That engineer I mentioned wrote a HEARO 999 review that almost nobody read, at least those in the audiophile community.
