Alright, after some more time with the B400, my opinion of them has greatly improved.
Fit is VERY important. I switched to the small silicone tips, and with a deeper insertion the bass gets much better.
Speaking about fit. The cable slider, which I had missed the first time around (seriously, I'm ashamed), helped me greatly in fitting the B400. This IEM has fantastic ergonomics for me. The earpiece sits flush with my ear, the nozzle's angle is pretty much ideal for my ear canals, and the tips go in effortlessly.
I only have two more problems to solve:
1) I simply can't see how the provided Comply tips could ever fit on the B400's nozzle. Seriously, it's impossible. The nozzle has a bulging end that helps silicone tips stay in place and not slide off when the earphone is removed (great idea!), but the plastic cylinder in the Complys is way too stiff to accommodate this.
2) My ear canals seem to be a special case. Smooth, single-flange silicone tips will always slide out of my right ear after a while, and my right ear also requires a deeper insertion to achieve a good seal and fully feel the bass. I really need a double flange to get a durable seal. I still have a pair of Ety triple flanges, but the hole is narrower than in the B400's own tips. Any suggestions?
As for sound, the B400 is starting to grow on me. I still find it a little dry, and it can get fatiguing after a while. I'm used to, and definitely prefer, a slightly smoother sound. Detail is excellent, but the soundstage isn't very expansive and all that detail gets kinda compressed, which may contribute to fatigue at higher volumes.
The great thing about the B400 is that it sounds almost the same as the old Westone UM2, a favorite of mine some years ago. The amount of detail, slam, impact and bass is almost identical. I take back what I previously said about bass: it's there, definitely, and maybe even a little too much with certain recordings. No basshead territory though: the B400 does clean, quick, powerful bass that doesn't bleed into the mids and doesn't overwhelm the other frequencies.
So far, I can definitely recommend the B400.