TL;DR: OpenHeart's Nightingale is a great budget earbud with similar tuning to Yincrow's earbuds; rougher around the edges and sibilant at times but tameable with minor EQ settings. Very easy to drive and sound great even with low-end sources but does take a stupid amount of bass boost to have decent low end. More flexible than other budget buds due to the MMCX connectors but cable isn't great and the MMCX plugs are incredibly difficult to remove, needs an MMCX tool.
I've been meaning to write a full post about OpenHeart's Nightingale which was mentioned here back in 2022 by several people but without any follow-up I could find. I wanted to compare it against the RW-2000 that I had foolishly left at home while traveling before I posted my thoughts about it. And after listening to the two side-by-side with my Qudelix 5K it has me re-evaluating whether I really need some of the buds I was interested in, as well as what I'm willing to pay for higher-end earbuds. While the RW2K has a much more refined, smoother sound with deeper bass extension and greater treble resolution, the Nightingale is at the beginning of the same region at least to my ears. Maybe I would notice a more profound difference with a higher-end source, but how high would I need to go to achieve a noticeable improvement? That's a deep, deep rabbit hole with no end in sight...
I honestly never thought I would get the quality of sound I get with this bud out of TRN's BT3S Pro or even the BT20XS, I thought I had hit the limit of both due to the lack of an amp and limited codec support respectively. They even sound great with a cheap
BT-50 neckband that I had written off as not worth the money. While it takes a stupid amount of bass boost (+40% in Poweramp EQ... yeah I know...) to have decent bass presence I don't notice any warbling or distortion to vocals like I do with the FF3/S when I push the bass too high. There is a noticeable point where the driver can't reproduce bass frequencies but I've noticed this with the RW2K as well though it isn't nearly as pronounced. Treble can be sibilant, especially for songs with prominent female vocals, but a -1.5 dB 4K 1.41 Q peak filter was enough to tame it without reducing vocal clarity. Even using the BT3S Pro I was noticing background instruments and effects that I hadn't heard with other earbuds, this was even more noticeable when using my 5K as the source. IMO the tuning is very similar to Yincrow's, with similar instrumental punch and depth that is sorely missing from Fiio's buds for me.
It's not all sunshine and rainbows, as there are multiple songs that I enjoy listening to with the RW2K that feel constrained and stuffy with the Nightingale. Normally there is a noticeable soundstage that while not as wide as the RW2K is still present that just disappears with these songs. I'm not sure if it's the vent placement, shell design/shape, or the Hiegi foams I'm using but it's rare enough that I'm willing to overlook it since everything else sounds so good. This is my first* earbud that has an MX500-style shell, so maybe that's a contributing factor? (*I do have the X9 which has a similar shell design but I haven't listened to it nearly as much as the Nightingale so I can't compare the two). I'm also not a fan of the included cable, it's similar to Fiio's FF3 cable that I absolutely despise though it's more flexible so it doesn't get in the way as much. And holy hell I've never had such impossibly hard to remove MMCX connectors, I bought an MMCX tool because of how difficult it was to remove even when trying to use my fingernails to get between the socket and the plug. I honestly felt like I was going to break the shell with how much force I had to use to remove the cable, if you get this earbud you need an MMCX tool.
This earbud has really opened my eyes to what could be possible by building my own earbuds, as I was afraid it would be a lot of trial and error to achieve a sound signature I like. But if a budget-oriented store like OpenHeart* can achieve this with inexpensive and (presumably) common parts with AFAIK no additional tuning or tweaking it makes me a lot more confident that it won't be a huge time (and money) pit. I'm also going to explore more options for custom earbuds as a starting point to DIY as it will hopefully give me a feel of how individual parts sound when combined into one package. (*Apparently XinYue is likely the original seller of this same earbud according to this post, I hadn't heard of XinYue until I was seeing whether anyone else had posted about this earbud. It doesn't seem to be nearly as popular with their customers, not sure why.).