Many thanks to
@HungryPanda for the tip on the ToneKing 200 buds. I picked these up, along with a couple of other pairs that seem well regarded. Here they all are in comparison to my original Monk buds:
A few points to note:
1) The Edifier P180 turned out to be extremely similar to my Monk Plus buds. They look like basically the same headphone from their frequency responses.
2) One thing I quickly realized is that measuring these is a bit harder than measuring IEMs. With IEMs, you can insert the IEM into the coupler at the same distance each time and get very consistent measurements. With earbuds, measurement variations arise from varying the pressure you apply to the bud on the coupler. If you clamp it on and press it down nice and tight, it tilts the frequency response towards the bass. The amount of pressure that your tragus and antitragus are going to exert on the buds against the ear canal opening are going to vary from person to person, and this makes a huge difference. Check out the effect of the P180 buds resting gently against the coupler, versus being clamped or held tight against the coupler:
This is a huge variation! My previous VE Monk Plus bud measurements were made with the buds clamped on the coupler, which I now think is unrealistic. For me, all these buds tend to just sit gently in my ears, so, IMHO, the 'correct' measurement above is the one in red, where the bud is resting against the ear canal with little-to-no pressure applied.
3) The ToneKings are incredibly well built and could be worth buying for the detachable mmcx cable alone! I found one problem with the mmcx cable though - the socket is positioned very close to the buds and the thick rubbery coating on the mmcx plugs didn't let the buds sit that easily in my ears - they just got in the way. So... time for an exacto knife and some heat-shrink tubing...
This still wasn't perfect, but it was much better. As a bonus, I can now tell the difference between the L and R buds without needing a microscope
4) The foams make a
huge difference to the FR. Donut vs full-foam, porous vs dense foam, all make a big difference, I suspect, mainly because the density and thickness of the foam dictate how close to your ear canal they get and what kind of seal they achieve. The Monk Buds and Edifiers have fairly thick but porous foam, which holds the buds farther from your ear-canal opening and exaggerate the high-end. The ToneKing donut foams are awesome - in fact I'm now using them on the Shozy buds (measurements of the Shozy buds above used the red ToneKing donut foams).
My conclusions... I love the ToneKings. They're awesome. However, they seem a bit more treble-emphasized to my ears than would be indicated by the graphs above, and I suspect it's because the cable connectors get stuck against my ear cartilage and don't allow the buds to rest quite as snugly as they do on the coupler. They're my second favorite. For me, first place goes to the Shozy BK buds - they have the best combination of extension at both ends, fairly neutral sound signature (no very exaggerated resonance peaks) and a decent amount of low-end, and even a little bit of sub-bass. But all these buds are very good value-for-money performers. They'd put a lot of expensive, full-sized over-ear cans to shame with these frequency responses.