You can experience the same thing using universal tips while getting a perfect seal too. Just use a bigger tip and insert any IEM shallow and you should notice less bass. If you click on my sig on H-fi you can see how many IEMs I've gone through. It's insertion depth and seal that matters. If you find pics you can see how some ER4 sleeves are made to accept the ER4 deeper while sleeved for the W4 are pushed surtehr out because they are slanted nozzles just like the B2/DBA. Make a custom sleeve that keeps the IEM six inches away from your eardrum and you'll notice the absence of bass.
ah, makes sense... I hadn't seen it this way, as audiologists always tell me it's tough because my ears are relatively shallow - must be my overall character trait ;-) but I never knew whether they were specifically talking about getting a hold for the IEM sleeve, or about the whole passage to the drum too...