The Acoustune HS1551 CU has a warm, natural and balanced sound across the spectrum with nice dynamics and a very good bass response, together with good imaging & staging capabilities. It definitely does impress for a single dynamic driver with its musicality and resolution combined. The work they’ve done on that driver has certainly paid off by the looks of it. That smooth, fatigue-free and warm sound put a smile on my face through my listening sessions.
I used the Sony WM1A from SE output for this review with the Direct Source mod. The Sony ZX2 and Shanling M3s are the other DAPs I tested it with. I used foam tips to achieve a good seal with the earphones.
Bass is the most impressive part of the Acoustune HS1551 CU with its good rumble, decay and fullness. It of course has a certain density as expected, as well as that spacious sounding bass that I love in dynamic monitors. Bass can get deep, can hit hard when demanded, and doesn’t congest the stage with its presence. The texture is very good and overall resolution of lows is successful for the price level.
One of the the best parts is the quickness of the bass. It’s not slow in any way and that feels great with some rock songs. Of course it doesn’t have the same quickness level of some high level balanced armature monitors. But I think it’s very good for a dynamic driver. I haven’t heard any kind of distortion with the bass either, as it is nicely controlled at all times.
This bass response can be a little too much or just slightly boomy for analytical sound lovers, especially with far from ideal pairings. But if I speak for myself, it is impressive and very rich. It sound lush and bass lovers would love this earphone for its sweet, full and warm bass. Reference & analytical guys wouldn’t like it though and that would be totally understandable as everyone has different tastes. Just bear in mind.
Mids are quite rich, musical and warm just like the bass, but it again doesn’t sound perfectly reference. Is it bad because of that? No way, it plays the mids very natural, effortless and without fatigue. It sounds very organic and coherent across. It’s just the tonal accuracy that I’m talking about. The timbre is on a little higher note when I compare it to other high end monitors. It’s not a big problem, and I admit it gives the earphones a nice musicality and adds a little fun to the mix.
Other than the tone, I don’t have complaints for the mids. They separate from the bass nicely and thanks to the overall midbass control, there’s no hump in lower mids. I noticed that it sounds very nice and caressing with Jazz songs. The DSD file of the legendary “So What” from Miles Davis plays incredibly smooth and organic from start to finish, especially the saxophone sounds highly musical. Upper mids sound a little closer than lower parts, but not overly doing so, therefore it’s all right and there’s nothing to worry there. Instruments have nice definition and transparency, especially for a dynamic IEM and that’s definitely not often to see.
Well, this is a warm sounding IEM so don’t expect a very sharp, emphasized treble response. Treble on the Acoustune HS1551 CU prefer to be out of the spotlight, keeping their rhythm and accompanying the rest from the back of the stage. This is just another thing that relies on the person’s own taste. The extension of trebles is not like the multi BA monitors that you listen to, which costs near 1000$.
The highs are again very natural and integrated with the rest of the spectrum. They don’t try to show themselves in an unnatural manner, they just keep their space nicely instead, and don’t attempt to interrupt the instruments. The only downside in my book is the overall extension of highs, which are cut early most of the time. It’s not very transparent on that area so the lover of cymbals wouldn’t be impressed overall. Other than that, the positioning of highs are close to perfection, and again the separation from the rest is very impressive for a single dynamic monitor.
The sound stage is very nice with the HS1551 CU, sounding both wide and deep. That’s the thing I love with dynamic or dynamic / BA hybrid creations. Most of the monitors which have a dynamic driver sound vast. That’s one of the qualities of this earphone. You’ll love the airy staging and Jazz recordings sound particularly good with it. As a Jazz lover, I fell in love with the imaging, wide stage and overall positioning of the elements. Combine it with its warm, organic and highly musical approach, you have a winner for warm sound lovers like myself.
Resolution is good, but not the best of course. If you look for the absolute detail and resolution, look elsewhere. If you’re fine with “good” resolution and if you put more importance in musicality, then it could be your dynamic monitor. It’s slightly veiled overall but don’t forget that I’m comparing it with the IEMs that cost double the price. Transparency is very good and satisfying on the other hand, giving the instruments the opportunity to shine in their particular area in the stage.
The separation of the 3 main regions of the range is one of the earphone’s strong suits. But, they’re not too disconnected so you have a consistent and coherent sound as a whole. Compound that trait with a nice sound stage and stereo imaging, as well as the good musicality and rich tuning, you have one of the best blends in the market for the lovers of warm signature.
In terms of matching, I recommend something exactly like the Sony WM1A with a neutral sound with good sound stage and good bass control. Bassy and congesting sources wouldn’t be fair for the Acoustune HS1551 CU. Something like the Cayin N5II would match nicely as well. Another DAP I think that pairs well is the Shanling M3s in that regard, with its very balanced sound with just a slight warmth. Still, the most important thing is the bass. The lows of this monitor are very impressive but can sound a little boomy and loose with an inferior source, and the overall presentation can get unpleasant.
From its design, R&D and to the overall sound, the Acoustune HS1551 CU is a very unique monitor top to bottom. I absolutely loved it as you probably guessed from the first part of my sound impressions. It’s warm, highly musical, rich and organic. Its coherent nature and overall connected sound makes it one the best options for audiophiles looking for this type of a monitor. I assume Acoustune had a certain tuning in mind when designing the HS1551 CU, and I must say the result is great.
The amazing build quality and rich content in the box lives up to its price range as well as its sound. The isolation wasn’t ideal for me but you can always roll some tips with trial & error, but I don’t have a huge amount of free time to try everything, so I stuck to the foams which usually work very well with my ears. The supplied cable is one of the best I have seen including several aftermarket cables, let alone stock ones that manufacturers usually provide with their IEMs.
I surely recommend the Acoustune HS1551 CU to warm sound lovers.