Summoned by dweaver, and here I am! Unfortunately I have not heard the Eterna, so half of the initial discussion is out the window. But, I would have to say the CKS70 is the poor man's IE8 due to sound signature of the mids and bass. Of course, the IE8 outclasses the CKS70 in every way, but they sound similar except the treble.
Best bass in an under $100 IEM (and the IE8)? Depends on what you want out of your bass. Fast, warm, big boom, punch, etc.
Denon C710: Might have the most deep punch, and is fast. I often found it had too much punch for non-electronic music for my liking. I didn't like getting punched in the head while listening to a mellow Sarah McLachlan song!
Cyclone PR2: Turns out mine were broken, as the right side would lose it's sound after a short period of time. When they worked, these probably had the most overall bass, but it didn't seem well controlled with the sources I paired it with.
Mingo WM2: Huge bass that seems to be there when I wanted, and not when I didn't want it. Very warm like the IE8, but more deep bass and a little less mid-bass. Overall has a similar balance as the IE8 from bass to treble, but the mids are more forward. The bass isn't quite as fast as the C710, but it has more reverb. These are better controlled than the PR2.
CKS70: I didn't feel it held an advantage over any the WM2 or C710 in any way. The WM2 bass seems more detailed, and I remember the C710 bass being very detailed as well. Where the CKS70 shines is perceived presentation width. The bass is good, but not the be-all-end-all I thought it may be. It is tighter than the PR2, but also has less bass. I would have kept it (well, I still actually have it sitting in a box waiting around), but I like a good amount of smooth, detailed treble, which this didn't have.
IE8: The IE8 with the bass knob at max can pump out tremendous amounts of bass, but there is so much mid-bass it lowers the impact of the deep bass, which is the major difference of the IE8 and the rest. To me, this is an issue based on my preference. But the IE8 surpasses all the other IEMs I mentioned in every technical category, space, width, detail, treble extension, transparency, etc.
Thunder: The Thunder doesn't have the most bass, nor the fastest, nor the best controlled of these IEMs. What it has is what I think is a fantastic balance across the spectrum that makes all genres sound great. But it does have plenty of bass when the song calls for it.
M5: The M5 had a lot of bass, close to C710 and WM2 levels, but not quite. And the rest of the spectrum left something to be desired in comparison as well as the overall space.
Turbines: While I thought the Turbines were overall good, and I did like the bass and mids, I didn't feel the treble was all that great. And as far as impact, I would say it was a little less than the M5, maybe on par or a little more than the Thunder. For the price at the time ($150), there really wasn't much else that could compete, but now, even at $100, they have some stiff competition.
And throw in the source factor. Are you using a DAP, and if so, which one? Maybe an amp, and again, which one? Each source can and will change the sound of the IEM. For example, I think the IE8 does much better with an amp, but the WM2 does quite well without an amp.
Not sure I cleared anything up, and maybe muddied the waters even more, but it comes down to personal preference and knowing what you prefer! And I prefer to have many IEMs I can choose from!