Just auditioned the FD01 again on the SS nozzle. I do think the sound signature is a u-shape, with midbass and lower treble emphasis. These are good rock and metal iems. The bass is quick and tight, subbass extension is ok but it should not be the selling point. I find that the bass does not contain much texture but it compensates for being slightly deeper and it resonates within my ears. There is a good "airy" feel on the bass which gives it a more unique timbre. It is not warm or distorted/colored - something I look for in bass.
The lower mids were reserved and upper mids was only very slightly positioned behind the bass. The tonality is accurate but it won't satisfy distortion guitars. For vocals, I find them to be a little thin and bright. You can still hear the singer's power but it is not creamy and thick when compared to the CA Andro.
It is the highs that stand out most in the JVC FD01. There is a nice amount of air in the treble and adequate sparkle. It's actually pretty good resolution. Do note that there is a peak in the treble. It helps the IEM sound less fatiguing. The treble sounds really distinct from the bass and mids - almost like it's stealing the show...it can be perceived as being rougher and edgier, as compared to Andro's smooth sound.
Instrumental separation and layering is above average. I can tell instruments apart and it does not sound as confused on messier songs. I think the bass, mids and highs being distinct sounding from one another helps. Due to this, you will not find it as smooth sounding. Imaging is slightly above average. When it comes to soundstage, width and height is honestly average or can be underwhelming when you compare it to similarly priced options/competitors. It does have some stage length to it though. Detail retrieval is excellent for its price...which I think that the clean background helps play a part in. The IEM is rather revealing & transparent actually.
In terms of construction and comfort, I tried using them over-ear. They are SO much more comfortable like that. It is still heavy and sturdy but it being over-ear helps with hotspots.
Overall, this is a bright IEM that has slightly more mid-bass presence to satisfy the modern crowd. The vocals/mids not being full-bodied can deter some from liking this IEM. It hits a good mix between being analytical & fun sounding.
Comparisons:
Mee P1: The FD01 is a direct upgrade imo. Has a snappier bass and higher resolution treble. Mids sound close...but the FD01 gets the tone, accuracy and cleanliness right. The Mee can get confused on busier/messier songs, whereas the FD01 will try to layer and unpack it.
Oriolus Forsteni: The Forsteni has an elevated and airy trebles, similar to the FD01. Forsteni has better treble imo, as there is a better impression of space. In terms of bass, forsteni packs more texture, depth and even more slam. Forsteni's mids are slightly veiled but they are full-bodied. The FD01 wins the forsteni in terms of imaging and layering imo. But in terms of soundstage, Forsteni seems to have a larger one as it is airier. Forsteni is slightly warm as compared to the FD01 where the FD01 sounds brighter and cleaner.