Its very noticeable. I have the Club sound and, if I don't EQ it in, there is almost no sub bass on unit. It took me awhile to get over that slight disappointment (its such a drop from the demo units). Its so missing that I stil wonder if I got a defective pair/tuning. But I can't get that looked at till after Covid restrictions are lifted.
I am not saying this is you guys' cases, but to fully enjoy Just Ear one needs some understanding of what it is.
First I have to say (mine is the LiSA variant)
1. demos have good sub-bass response because it is the ear tips that offers the seal hence the bass. As long as you have the right sized tips on you will have a consistent seal for the duration of the demo period (say 30 mins or so).
2. My own pair of Just Ear gives the same sort of sub-bass most of the time, but not always. There are conditions under which it underperforms.
Different to other CIEMs (I got a few) and different to UIEMS, the Just Ears relies mostly on the outer part (big part, or that part that rests in your pinna) including the part that JUST cling onto the entrance of your ear canal for fit AND seal. The protrusion that extends into your canal tends to be made a tiny bit smaller than the other CIEMs and not necessarily adheres to the wall of your ear canal all the time.
If I have listened to other iems especially UIEMs which I pair with a larger than usual sized silicon tips for a listening session or two then switch back to the Just Ear, I tends to find it underperforming, not just with much less sub-bass but also with less high extension or details in the mids. That doesn't happen if I have done the same with UIEMS that inserts deep and which I could pair with a smaller sized silicon tips or foam tips (foam tips cause less of this problem as it doesn't force your canal to conform to its shape). That also doesn't happen if I switch from other CIEMs (which undoubtedly fits great to my ears and won't bulge in the canal) to the Just Ears.
I have concluded that ear canals' sizes and shapes would be slightly altered by larger and more rigid tips of UIEMs. While this usually have no bearings on usual CIEMs which relies equally on both the pinna part and the canal part to fit and seal, it affects the performance of Just Ears as the size and shape to the entrance of the ear canal is very important for the seal which in turn is very important for sub-bass output.
This is only my experience and YMMV. The above may not apply to you. You may be a one iem person (which I doubt there's any on head-fi) so the above is just BS to you. But if it happens that you are one to rotate heavily with a large iem collection like I do, you may try listening to the Just Ears a few days straight or, if you want to rule out the effects of brain run-in or adaptation, try ONLY listening to CIEMS and Just Ears for a few days straight.
After I draw my own little conclusion, I avoid using large rigid tips altogether for good and my Just Ear has since been a true world beater.