I've played around with all kinds of mid to slightly higher end earphones and headphones. Of the ones discussed in this thread that I have either owned or still own. My setup is a Sena 10s going to a PA2V2 inline amp, and then into these earphones.
Etymotic ER6i: They were super clean/clear, amazing for voice/podcasts/horns/brass, but maybe too much so and lacked bottom end. They were so "crisp" they really highlighted low quality (bit-rate) audio and static and electrical disturbance to the comm unit. So they didn't exactly work great with BT streaming. This is just my opinion, I know many people that think these are some of the most "hi-fi" earphones they have ever used. In heavy/regular usage, I ended up damaging the wire as it entered the earpiece. I got less than a year out of them. I used the supplied triple flange (I believe) silicon eartips. They worked great for passive noise cancellation, and minimal casing meant they were easy to keep in the ear getting the helmet on. On the PA2V2 with some of the highs filtered down a bit, sound great . . . really adds some body to the thin/accurate feel of these. Would not buy again as they just weren't a good match for my sound preference.
Shure SE215s: More balanced compared to the Etymotics. However, for motorcycle usage, I do not like the over the ear "memory wire" type fitment. I find it harder to keep in my ears as I get my helmet on. Over ear style fitment just doesn't work for me and helmets. Benefited very well from the PA2V2. These were my everyday earphones I'd plug into my smart phone or a laptop or something like that, until I picked up the Sennheisers, which in my opinion are a better day to day run around earphone as well as a better motorcycle earphone because the Senns have a smaller casing so it's easier to keep them in and get your helmet on.
Sennheiser CX 300B MKII: These have bass the Etymotics can only dream of, but I bet some audiophiles will say these also don't compare in terms of "clarity". To each his own. Basically if you like your EQ setup with a little bit of a "disco smile" or a little heavier on the bottom end (as you can always EQ out a bit of the top end, but you can't make earphones suddenly bassier), these are great earphones. Sound great directly plugged into a smartphone/laptop, etc. Small casing, easy to to keep in the ear getting helmet on. Assymetric wire setup, short on the left, long on the right so siamese point is on your left. May or may not work with your setup. Seemed to work better in my old jacket where the last elasticated wire taming loop for the wire in the jacket was lower and further away. On the new jacket, this setup tends to be more prone to tugging my left ear loose as the last elasticated wire taming loop in the jacket is right at the collar. Single flanged silicon ear bud is less effective than triple flange or foam earbuds so these have the least passive noise cancellation of the earphones listed so far. Can definitely tell the Shures are more ruggedly built. I get the impression it's only a matter of time before I tug (likely on the left) ear piece too hard and damage the wire/connection.
Plugfones (the ones that look like yellow triple flanged earplugs): These have surprisingly better audio then they should. And the triple flange really blocks out the noise. They are a joy to use in a loud environment, i.e. a construction site. However, I found these at least as fragile as the Etymotics, and these also did not last me long. Also, I didn't like them for motorcycle use as the end that sticks out gets pushed on by my helmet and tends to drive the plug into my ear causing pain. Seemed to have the least audio improvement with the PA2V2.
Sony MDR-XB50AP: Great bass. Great overall sound. Very reasonable price. Sound great with or without the PA2V2. I like these at least as much as the Sennheisers, and I mostly got them so that when the Senns break, which I see coming, these will be the backups. Don't work well with helmets as they have a large casing, or at least one that sticks out. But at ~$30, they're pretty good everyday earphones. At their price, quality, and slight emphasis on bottom end, these are a fav of mine, just not for motorcycle/helmet use.
What have I been running lately? I've been experimenting with earphones from overseas. Most of them with dual drivers in each earpiece. Some have been terrible, some have been at least as good as the Senns, and with the 10S and the PA2V2, I actually prefer some of them to even the Shures when on the PA2V2 and riding. Some of the dual driver units can handle a tonne of power, and PA2V2 works great with these dual driver units, especially if you're more of an analog sound guy vs more technical/clean. They ranged from $4-25 US. My current favorite out of the ones I've tried look like this, and I believe I paid $4 US for them IIRC. I think you can find them on Amazon.ca from $15-20.:
https://ae01.alicdn.com/kf/HTB1CtO1aKUXBuNjt_a0q6AysXXav.jpg
And I could be wrong but I believe the branding was QKZ. They can handle a tonne of power before starting to distort. I use memory foam ear buds on them as lately I'm finding them more comfortable than triple flange and similar in passive noise cancellation.
https://ae01.alicdn.com/kf/HTB1q6SlcHZnBKNjSZFGq6zt3FXaM.jpg
