I have very similar experience with this headphones. I finally checked around 40 tips untill i found a pair that made the sound acceptable. Still not better than some old IEMs that i have. So easier is just to return the IEM and search for other that fits you better.Just made an account to comment on this as well. I received mine yesterday and I have the same issue. The bass is really weak while the higher frequencies sound to prominent. I've tried with all of the included tips as well as some much broader ones I had at home that definitely do close the ear channel properly. I think the best experience so far is with the blue tips included here, however it still sounds "flat" compared to my prior in-ear headphones I've used for 13+ years now (Philips SHE8500), which were ironically even more budget and yet had a far superior sound.
There is one thing I've noticed: when I put both of my hands over the ears, the bass increases quite noticeably. So is it a sound isolation issue? But it cannot be the tips. I've tried quite a few other, larger ones I've had at home. So is it the low insertion depth as some pointed out which makes these ones so dependent on anatomy?
Well, anyway, I've ordered the Zero Red and see if those do a better job for me. But yeah, the Zero:2s turned out worthless for my ears it seems. Really don't enjoy the thin sound. Probably I'll send them back to Amazon or something.
But what i've learned is that tips have far more influence that i thought and sadly that i can't predict the tip effect based on shape, size or material. Also the IEM shape, seal and comfort are not less important than the measured FR