My Ziku HD X9 has a noticeable subbass roll off too, though I don't have objective graphs/measurements to compare it with the X10. I'm a basshead so I was a bit annoyed with that at first, but I now use super bassy IEMs with the HD X9 to even out this roll off. I think it's more obvious for bass forward music like EDM. Maybe not so if u are not a basshead or listen to more neutral genres like classical.
I think the battery life is supposedly longer in the X9 than the X10, but I would think in other areas the X10 should be improvements over the X9 (such as op amp etc).
You’re 100% correct about counteracting the roll off with super bassy iems. You can also do this by changing the op amps. Not to mention really good digital filters that can also get those roll offs. Ideally, the roll off should be lifted a bit at 100 hz but I’ve never seen it flatlined without any lpf or hpf. I have heard they make them as well as iems that are flat.
I have found that AD8066 has the best bass nuances in all areas of bass in your music if you’re into op amp roll and can so on your dap. In my opinion, the entire line of Analog Devices of op amps are really good for bass and those amps sound alive (Imho).
Personally find too much bass muddies the mids and is fatiguing for long listening sess.
My motto has always been: the bass should always be felt not heard
However, the past I’d say 20-30 years bass has really become such intricate part of musical expression in nuances inside the sound of those frequencies (accent markings, grooves and melodically) in genres like Electronic, dance and hip hop music so I get it!
When the music calls for It like say a Kendrick Lamar track or a drum and bass track and I wanna hear all the bass nuances I’ll pop in my bass rig that suits those music styles (iems, op amps, etc) in my daps but that’s only certain occasions and moods.
So in my opinion it’s not a either/or and it shouldn’t be a “deal breaking” for bass heads if the dap is steeped and sloped at 100 Hz and you can’t always go by charts alone; there are many variables.
I’m also sure there are daps out there specifically for the bass head niche market or through word-of-mouth in the audiophile community.