Got some impression two months after, and I feel they are still top notch.
These take my top spot for the best (and most versatile) dynamic driver IEM across everything I've tried, including TOTLs like the CFA Vega, beyrdynamic Xelento, AK T8ie Mkii, Sennheiser IE800, and some hybrids like the Fostex TE100, CFA Dorado, Dita Dream/Brass, Oriolus Mkii, Sony EX1000 and the Vsonic GR09.
The CFA Vega feel too overly bassy for me, and lack the intimate mids that the HS1551 does so well.
Beyerdynamic Xelento and The AKT8ie come close in performance, are faster and have great extensions on both ends, but also suffer from slightly recessed mids. Just slightly, though, I'd pick these as a nice second place for best DD iem.
Sennheiser IE800 is fast and pretty expansive, but also can be quite a dark mess compared to the Acoustune.
Sony EX1000 is good, but can be a tad too bright for me. Might sound thin to some people too.
Vsonic GR09 is similar, neutral with a bass tilt. Good resolvong iem, also fast, and up to preference here. Both are good, but I prefer the 1551
Dita Dream sounds similarly dark/warm, and works on very specific pairings and music to sound good. Dita brass is similar, less dark, and very mid-centric.
For the hybrids, the Dorado is dark as hell and lacks mids too.
The Fostex TE100 is nice and warm, but has an odd treble roll off that doesn't sound too nice.
The Oriolus Mk2 is incredible, warm and somewhat dark, one of my favourites, and can compete with the 1551. Close tie between the two.
The Fidue A91 Sirius is detailed, but slightly V and can be boomy in bass too.