Personally, I find the a8k to be the higher performing iem, but they’re both pretty different in focus. Both are also very picky on source and fit lol. Imo the 900 is more intense u shaped, a8k is more neutral bright leaning.
I actually feel that both have a strength in treble, in a different way. The a8k focuses on a more even but elevated, sweet, and very airy and open type of treble, where the 900 is a more forward, energetic, and intense sort of treble that while a bit less even can still be refined and nuanced. I think both can have pretty awesome treble if you get the source right, and both are pretty prominent in the area. Not painfully or distractingly bright, as long as you have a good source that is lol. But I’d say go a8k for more even, sweet, airy, delicate but pretty prominent treble, but 900 for more energetic, edge, crafted treble response that’s less even and more fun per se. I do feel as though the a8k is both my preference and higher performing here, but I could see someone liking the more intense and immediately fun and energetic approach of the 900 as well.
Bass is where things significantly deviate, the 900 is a much different focus than the a8k, the 900 is much more heavy handed, boosted, and forward. If you’re after bass quantity then definitely the 900 is more satisfying. It’s also still fairly technical in the bass too, great texture forwardness, great slam, solid balance of dynamics. The a8k on the other hand is a much more neutral, slightly lean, more nimble, speedy, and nuanced bass, that while it lacks in slam and pure grip provides in awesome nuance and lower level focus with a slight bit of roundness and sweetness as well. Both very different focuses, I honestly could go either way here depending on what I wanted, I do think the 900 feels more immediately enjoyable in this range but I think despite this I was more satisfied with the a8k here longer term, but depended on what I would listen to.
Midrange imo is really where I think I could say I just prefer the a8k most times, the 900 is a bit recessed and it takes a bit more of a backseat here, depending on the pairing to a significant degree or a more mild one, but still recessed either way. I think the a8k sounds more correct here to me offering better balance in tonality, better dynamics, better timbre, and better texture and resolving power. If you have a focus on midrange, a8k is just what I’d consider more.
For technicalities I personally feel the a8k is just more resolving and better presented in that resolution and texture, the 900 in comparison can sound force fed. Dynamically I do feel as though the 900 is more macrodynamic focused, still good in micro but more macro to me, the a8k is more micro focused and while it’s not lacking in macro, it’s a bit more reserved there. I do think one trait the a8k has that the 900 doesn’t accomplish as well is liquidity, the a8k just sounds more coherent and more smooth and liquid to me. Both have solid speed and separation with the a8k being a bit better in those regards, overall control and grip is superior in the 900 for bass but better in other regions with the a8k. Slam and impact is much more impressive on the 900 lol. I do think the a8k can come off as a bit rounded slightly softer at times. The a8k is overall also sweeter, where I wouldn’t consider the 900 to be that in most pairings.
Overall presentation and space is something where the a8k really shines imo, makes the 900 feel like it falls short. It’s much more open and blacker sounding in stage, and it’s stage is more immersive and grand while still feeling more organic. It’s got a lot of width but also has awesome depth as well. Just really impressive there for an iem. The 900 feels more traditional, I do think it’s focused more toward width than depth, a bit grand sounding, and sharp ish in placement, but I really think the a8k is just more impressive to me here.
Pairing wise, the 900 is actually more picky, if you get a bad pairing, it will show it more obviously than the a8k, you’ll get sucked out mids, overforward and harsh treble, and you’ll lose out on a lot of refinement and technical performance, The a8k is also pretty noticeable when you don’t feed it properly getting a bit overall bright and lean, but it’s not as immediately drastic and unpleasant as the 900 can get.
Basically, I think they’re pretty different iems for different roles. For myself, if I wanted something more intense and fun, I’d go 900. If I wanted something more refined and nuanced, I’d go a8k. Both can be organic but I think the a8k is more organic in the time domain and tonality.
. Apparently, it needs loads of burn-in which would have been nice to know when I got it (thanks for the heads-up M0N). Anyway, with the system I have running with the brise attached it’s honestly the best example of a well-synergized system I’ve personally heard so far. I frickin love every second of it!