First, let me 'fess up. These are the two best headphones I've heard in many years, like decades. I listened to some Stax Lambda Pros a good bit back in the 80s. Those were the best I heard then, and that was a long time ago. Aural memory being what it is, I certainly cannot compare those accurately to these. First 'fess. Second. Back then I could hear up to 18k, give or take a k. Now I top out at 10k; I can hear 10k just fine, both ears. Well, rolled off, just fine. I know this because the first thing I did was play test tones to begin this comparison, as a check both for the phones and my hearing. The 10k required a bit more volume than lower frequencies for me to clearly detect. Second 'fess. Third. I don't have the advantage of having heard any high-end headphones lately - I lack a TOTL reference. Perhaps I could, or will, think of several more confessions I might make. Yes! Fourth. I'm a dumb-ass, and I know this because I paid three times more for the 400i than I did for the 58X. So therefore I'm biased, right? That is a popular enough sentiment around here - if he paid a lot, it has to sound better. OTOH, I'm kind of pissed that Hifiman started blowing them out at sale prices about three weeks after I overpaid, and that stings a little, so let's just call that a wash, shall we? The Hifiman is a, what, $220 headphone? The 58X, $150, and a special deal from Sennheiser thru Massdrop - they even produced a new driver for it. Near enough the same money, near enough the same value. Or is it? So that's four confessions that beg the question: What do I care about a dumbass-who's-deaf-above-10k-and-hasn't-heard-the-best-HP's comparo of these two headphones? I'm so glad you asked. My answer starts with you can't even buy one of 'em - maybe they'd bring the 58X back for another drop, maybe not. The reasons not to care are mounting... maybe I should just stop here. However, I'm simply not that smart, remember the dumb-ass thing? But I can think of a reason or two for you to read on. If you are thinking of purchasing headphones and reading this, you probably haven't heard the TOTL models, either. And many of you are as old as I am, and probably don't hear much higher in frequency than I do, and some of you less. If you are in this group, do you want to read what some young whipper-snapper with 20k (gold?) ears thinks of the sound, or would you rather hear from someone who shares your HF deficits? Think about it. Okay, you'd want to hear from both. Fair enough, me too. And the last reason is that some of you are dumb-asses, too. I happen to know this, being so able to spot the trait in others (takes one to know one). Well, I told you about the test tones, and how my listening acuity fared, what about the phones? I expected that the Sennheisers would have deeper bass extension, but actually they both did well from 30hz to the limits of my hearing. No egregious noises, misbehaviors or anomalies noted with either pair. Both sounded smooth rather than peaky to my ear using sine waves, pure tones. I listened to Donald Fagen's Morph the Cat, the track, all the way through with both headphones, twice. I expected the 58X to go just a little deeper, but it didn't. It was first up. I thought they sounded very good, but not as resolving through the mids as my speakers. Still the muted horns that come in the second half the track sounded good to me, but not as good as the Silver 6. Same for the well-recorded vocal harmonies. The bass seemed just a tad soft to me, lacking the articulation to which I'm accustomed. Some people will like the slightly warmer sound, though. The HF sounded reasonably detailed, I thought. Then it was the 400i's turn. It won, hands down. Sorry, but that's just how it is. Immediately apparent was its advantage in the top end. More snap. More detailed. More better. Mids. Muted horns sounded more immediate and lifelike. Resolution was so obviously better that I wondered why I didn't recognize it while I was listening to the 58X, but it's mostly due to not using the 400i lately and that aural memory thing. Head to head the improvement was glaring. I know there's been a lot of hype around the 58X, saw the choices when I did a search, but didn't read any reviews because I wanted to come to this comparison fresh, and get it wrong all on my own, if that is what I've done. For all I know the experts are saying they're the best thing since sliced bread... but I doubt it. Their standards, or rather, their reference will be better than my 400i, and I can't see them swooning over these. Even the 400i's bass sounds better. If you pulled both out of boxes and listened to new pairs, you'd call BS on that. The 400i do have a long break-in until they stop sounding anemic, and even longer until they reach their full bass capability. I know, you don't believe in break-in, I just became acclimated to the HP. Wrong, get a pair and stick with 'em, don't return them when the bass sucks, and you'll find out for yourself. I don't know if it is because they are planars, or why the long break-in, but that's what happens. So the one you can't buy is the one you shouldn't. You lucked out. At current sale prices, the 400i is a steal, imo. The 58X is competent - it won't actively annoy you. Sound better than my $60 AKG-240S. If $150 is as far as your budget can possibly reach, you might consider a used 400i pair, but you didn't miss the deal of a century on the 58X, either. The 400i definitely gets a little closer to an electrostatic Lambda type of sound. Okay, in closing: I made my statements declarative, but I certainly don't claim infallibility. All this was my opinion, but it would read ten times worse if I kept throwing in IMOs or YMMVs every other sentence. Your experience differs from mine? Well, you might just be right, and I missed it. Or we both may just be right because we hear differently. BTW, are you a young whippersnapper?