but
Well, that is true when it comes to the DAPs of the same manufacturer. They generally try to make them better with each line-up
FiiO has X1, X3, X5 and X7. From those, X7mkii, the top of the highest line is the best sounding by a large margin to my ears. Basically, it has the best vividness, musicality, detail, clarity, extension and everything. That one is the best FiiO DAP. X5ii is the second after it in my opinion, if you're looking at the sound alone. X5-3 went into another direction entirely, it is ultra smooth and doesn't render cymbals with energy but rather with relaxation, making it unfit for metal music, so X5ii is immediately behind X7mkii. The first generation of X7 had some hardware issues and I cannot say where it lies, the sound was incredible, but the device was hit and miss, would work wonders for some, would be problematic to some, depending on your listening habits.
Shanling has M1, M2, M3, and M5 DAPs. Their M3s is the latest from the M3 line and M2s the latest from the M2 line. M3s is basically their mid-high end DAP, which features the same control and body as M2s, but it is larger and people said it has more vividness, detail and clarity. M5s is not released yet, so right now the only one from the M5 line is M5, which I don't know nothing about.
Now, M2s is the portable version of M3s, imagine that differences between DAPs are usually smaller than differences between IEMs or headphones, it is like adding the best spices to a food, not the base food, which is the headphone or the IEM.
It doesn't matter what you go for, all of them, even the first X1 form FiiO is much better than Clip as a DAP, providing a vivider sound, more clarity and more details along with much larger soundstage and better extension up top.
When it comes to M2s vs M3s, I didn't actually hear one, this is why I quote myself that it is seemingly better, and not necessarily better. I simply do not know yet. I have one incoming in a little while, and as a general rule it is the better version of M2s, so I am expecting it to have more clarity, details and vividness
If you need something ultra portable, I recommend M2s without fear, it is one of the few less expensive DAPs I included in my ER4-XR pairing list because it actually works very well with them. I cannot say about other less expensive DAPs without hearing them, but M2s is not to be looked down upon. As small and unexpensive as it is, it impressed me a lot more thunan I was expecting it to. I was expecting very little from it sound wise, but it does a lot. Again, try to pick the one that suits your needs perfectly, if you want something ultra portable, M2s is no joking matter, if you want to sacrifice some size and portability for an incremental increase in SQ, you can go for something larger. The differences are not as big between a small and a bigger DAP as they are between Clip and M2s for example, it is just us, more... dedicated music enthusiasts that try to get the best we can. ER4XR will sound extremely detailed with anything you plug them into, M2s will relax them a bit, X5ii enhance them a bit, X7mkii further enhance them