Was writing this as BlueSky71 was posting.. Yeah man, that high-mid/treble resonance thing is problematic!..
Volt- Not a more uniform response than the Etys, but more uniform response when used as I mention above (fully inserted, compressed both axially and radially), than as typically used with them not fully inserted and some uncompressed foam hanging out of the ear canal. Full insertion and compression maximizes bass attenuation while slightly decreasing mid attenuation and at least not increasing high attenuation further - so it makes the response somewhat flatter that way. That helps deal with overly loud subwoofer content which is a bane to me.
The Ety's and similar designs are flatter and great for not so loud stuff, but I frequently need/want more attenuation than they provide, especially of low bass. And some of them have a response that doesn't work well for me, as I get an upper midrange resonance thing with them.
I used to dream about the ultimate ear plugs combining physical occlusion (which attenuates more than well enough at mid and high frequencies) with active noise-canceling (albeit with sufficient headroom) as it can in theory attenuate low frequencies to a greater degree than can be achieved by an ear canal occluding foam plug, and an active "bleed" past the occlusion over which one had control of the level, making them useful at a far wider range of SPL levels.