These two cans may be one of the most pleasant, natural sounding headphone AKG has ever produced. Frankly, I haven’t heard the K601, but compared to the K701, K401, K240, K271, and even the K340, the K500 & K501 remains a long time favorite due to their natural sound. The question then becomes, why were they continued? It seems that part of the AKG corporate philosophy is to produce a great sounding headphone, then discontinue it, so that people who wants it can’t get it any longer. Pretty effective way of building a brand image. AKG: the maker of great discontinued headphones.
The K500 and the K501 is identical in shape. As a matter of fact, the housing and headband design is shared by the K400, K401, and the K301 as well. The donut shaped pads are made out of synthetic leather material, and people often upgraded the pads to either the K601 or the K701 pads. The K601’s pads are of the same shape as the K500/501, but it’s made of verlour. The K701’s pads are not perfectly round, but they have a shape that helps to put the drivers at an angle to your ears. Every one of these pads alters the sound slightly, and so it’s up to you to decide which pads suit your music best.
If the K501 is rare, the K500 is even rarer. Though I personally like the sound of the K501 better than the K500, (I’ll describe the differences a few paragraphs below), the K500 is still desirable precisely because they’re very hard to get, and the black housing to the gray headband color scheme is way cooler looking than the K501’s gray housing and black headband. Not to mention that almost every K501 that I have come across have peeling off model marking.
Before we move on to talk about the sound of the K500 and the K501, let’s talk about amplifier requirements of these two headphones. They are one of the hardest headphone to drive, next to the likes of the K340. Why? I have no idea, but the K501 are harder to drive than the K701 (75 ohms), as well as the HD650 and the HD800, and the K500 are not so far behind it in terms of amplifier requirements. Both the K500 and the K501 may be rated at 120 Ohms, but they are far more difficult to drive, than headphones with higher impedance ratings such as the HD650 (300 Ohms) and the HD800 (300 Ohms). Not only do they require a high amount of gain (roughly 25% more on the volume knob than the Sennheisers or the Hifiman HE5 Orthodynamic), but the K501 also requires quite a high amount of current. So far, the only other headphone that’s harder to drive than the K500/K501 are the K340, possibly the K1000, and some vintage orthodynamics. Some amplifiers may have enough voltage gain, and so you can get a good amount of loudness from these headphones, but that doesn’t mean that the amplifier can supply enough current for these cans.
When under current, I find the K501 to suffer more than the K500. These wonderful headphones suddenly lack bottom end impact, and I can’t help but thinking of the word “impotent cans” that can’t seem to get themselves up. The effect is more or less the same on both headphones, but is more pronounced on the K501. I really can’t give a strict rule on what amplifier specifications are need for these two, but I can tell you that the Dr. DAC Prime, the Lisa 3, the Heed Canamp, the Grace m902, the WooAudio6, and the Beta22 are some of the amps that I’ve found to work well with these cans. But please don’t conclude that you need expensive amplifiers to drive the K500/K501s, because apparently Dhani’s CMOY (on steroids, using a 19V power supply and an array of fancy opamps like Burr-Brown’s OPA552PA and Analog Devices’ ADA4627-1ARZ, AD825ARZ) did a great job on driving the K501. I really wouldn’t spend too much money on getting a premium, reference amps for these cans, but it is important that your amplifier can deliver the power needed.