According to Faust3D’s Stax Wiki, the SR-404 features a thinner 1.35 micron diaphragm, a jump from the 1.5 micron diaphragm of the SR-Lamda Pro and 2.0 micron from the SR-Lambda.
The sound of the 404 Signature is smoother than the SR-Lambda, but I can say that the biggest difference you find is the added body in the low and midrange. What’s surprising, however, is that despite possessing a thicker body in the low frequencies, the 404 Signature’s upper bass punch is actually weaker than the SR-Lambda, and it’s not as focused so you get less of a snap on the punch. It’s important to distinguish the difference between bass body, presence, and bass punch. You do get more bass presence and body with the SR-404 Signature, but less punch than the SR-Lambda. Lambdas in general are not a strong bass performers, and the weak SR-404 Signature bass punch can be a real deal breaker for a lot of people, including me who’s not a bass head.
Okay, the bass and midrange body is a very welcome addition to the SR-404 Signature, and suddenly you’re rewarded with a warmer, and more musical sound than that in the SR-Lambda. But, after long term listening, one thing stood out on the SR-404, and that is the treble remains thin sounding. I don’t understand how this can be, but I’m just describing what I’m hearing. So, you have nice body on the bass and midrange, and suddenly from the upper midrange areas and up, it’s thin sounding. This transition is fairly unnatural and is unpleasant to the ears. The SR-Lambda Pro might have a thinner overall sound, but at least it’s free from the sudden “jump” in the body. I think the sound signature of the SR-Lambda Pro and the SR-404 Signature is fairly unforgiving to bad recordings and bright sounding DACs, mainly due to the thin sounding treble in both cans.
As you move up to the SR404 LE, you can feel that not only does real leather pads feel nicer on the skin, but the drivers are sitting closer to your ears. On my ears, the driver cover is actually touching the edge of my ears. Taking a measuring caliper and measuring from the middle of the pads to the driver cover, I get the following measurements:
SR-404 LE: 17.1mm
SR-404 Signature: 19.4mm
SR-Lambda Pro: 21.2mm
All the pads are still in a fairly new condition, and so we can confirm that Stax has moved the drivers closer to the ears with each newer model.
The Limited Edition is definitely the best sounding of the three. On the Lambda Pro, you get a thin sound throughout the frequencies. The SR-404 Signature improved on that, but only gives you a fuller body sound on “half” the frequency range. Finally, on the SR-404LE, the sound is full bodied throughout the frequency range. What’s more, bass presence is more felt on the SR-404LE, and bass punch is stronger and weightier than on the Lambda Pro. The sound is also smoother than what you find on the SR-404 Signature and the Lambda Pro. Though all three headphones display sibilance whenever they’re present on the recording, the SR-404LE seems to handle it best than the other two, while the SR-Lambda Pro is the worst of the three. Easily, the SR-404LE is more musical and more forgiving of the source quality than the other two. Let’s just hope that the SR-404 Mk2 will come with the same sound that the LE has.
The differences on the headphones may not be too evident on a short listening session or in a meet condition, as all three headphones still carry a similar lambda sound. But, if you do buy and own any of those cans, long term listening will reveal to you those differences in sound signature, and at the end, you will wish that you had bought the SR-404LE.
Yet the SR-404LE is not perfect and without limitations either. Though possessing the most desirable sound out of all the Lambdas, the SR-404LE still come with all the limitations that a Lambda has. The unnatural upper midrange is still there, as it is in the SR404 Signature, the SR-Lambda Pro, and the SR-202. Some people refer to this as the upper midrange glare, and although that may not be very descriptive, I really can’t find of a good description except that the upper midrange has “issues” that doesn’t sound natural. The SR-404LE does tend to “cover” this issue better than the others, and it’s less evident than on the other cans due to the fuller overall body, but the issue is still there.
I also expected to hear improvements on technicalities such as treble and bass extensions, detail and imaging capability, but unfortunately, I found none. Of course the slightly different presentation, the amount of bass on each headphone is different, and that may “trick” us to hear that one headphone has lower bass extension than the other, but the fact is none is more extended than the other. The imaging capability is also very similar throughout the three, although the SR-Lambda Pro does have the most airy sound. The instrument separation is superb on these lambdas, but the soundstage image only exists on the left, right, and top, with a big void on the front. It’s fairly a two dimensional image as well, as there isn’t much depth to be found. It’s also a pity that the timbre is not as good as what you would find in dynamic headphones such as the HD650 or the Grados. Since a lot of the music that I listen to is lively recorded classical, timbre is very important. I’m also bothered that the superior speed of these lambdas tend to cut the delay unnaturally short. This may not be an issue with some other music though.
It seems that the Stax Omega2 is the answer to all the common problems that plague the Lambda, but the heavy amping requirement on top of the premium price on the headphone itself has been quite a discouragement for a lot of people to jump into the Omega2. And of course, you have the ultimate end-of-all Sennheiser Orpheus, but let’s not talk about that.
For now, the SR-404LE remain as the most ideal, semi-affordable, solution for electrostats lovers, and I would gladly recommend the SR-Lambda Pro for people wanting to get to electrostatics.
System used for review:
Headphones: Stax SR-Lambda Pro, SR-404 Signature, SR-404 Limited Edition
Amplifier: HeadAmp KGSS
DAC: Neko Audio D100 DAC
Cables: Zu Mother Power Cable, Kimber PK10 Power Cable, Transparent Musiclink Balanced Interconnect, Stereovox HDXV Coaxial cable
Others: Trends UD10.1 USB to Coaxial converter