For people talking about the bass, try
this seal test, it's an alternating 50Hz and 500Hz tone. If you have a bad seal the 50Hz tone will be quiet, if you have a good seal it will be clearly audible.
For me, I actually get the best seal with the stock Spring Tips. This was a bit of a surprise, as I'd actually got Spring Tips before and found they didn't fit/seal well for me on many IEMs, they were particular. But on the Chu they do. I think Spring Tips tend to work well with longer nozzles, and the nozzle on the Chu protrudes quite a bit further from the body compared with for example the Aria. (Comparison photo below.) I tried SpinFit CP100 and Sony EP-EX11 on them and they didn't seal well at all. Whizzer SS20 which are a short wide bore tip like the Spring Tip worked better, but best for me actually turned out to be the Spring Tips. I didn't expect this, I thought I'd likely use CP100 or EP-EX11 on them, particularly given the relatively small amount of bass shelf.
With the Spring Tips and a good seal, I'm getting good bass and in particular very good sub-bass extension. This isn't a significantly bass boosted IEM, so it's not massive, but it's certainly very much there. As for the sound in general, wow, I'm very impressed. Initial impressions are that this has substantially more clarity than the Aria, going back to the Aria it sounds quite muddy in comparison. Some of this is the tip- the Aria actually comes closer if I put Spring Tips on that. The Aria does have more bass though, as well as slightly more relaxed upper mids and treble. Upper mids and treble are very present on the Chu but I don't find it shouty. It's very close but slightly under Harman 3-7kHz. The smoothness maybe helps here, from the graphs it seems to be particularly smooth and lacking in peaks through the whole midrange and lower treble. It sounds very natural, maybe a little bright but natural. For $20 this is exceptional, even priced around the Aria I'd say it's worth consideration if you were looking for something with a less bassy, clearer signature.