I'll give away a little bit of my secrets right now before I post more detailed impressions - with what I remember off the top of my head. I was very surprised in finding that the Primo 8 can make my un-amped Westone 4 sound a little too laid-back and more veiled in direct comparison, similar to when comparing a more neutral and detailed HD 600 or HD 800 to the HD 650 which needs a more powerful amp to "wake up" and not sound dull. The RE-600 un-amped are even more laid back than W4, almost to the point of being dull in comparison. Things do improve when the W4 and RE-600 are amped.
Or, the comparison might be like comparing a HiFiMan HE-500 to an original LCD-2 rev1, where the LCD-2 rev1 is a bit more laid-back than HE-500 - the HE-500 and newer LCD-2 rev2 are just more vibrant, alive, and present with vocals and acoustic instruments (like Primo 8) than the HD650 or LCD-2 Rev1.
The Primo 8 sounds more alive than most universal fit IEM that I've tried. While the UM3X is closer, it's more forward sounding and less spacious sounding. I also find myself wanting more warmth from the UM3x with many amps or sources, but I don't find the Primo 8 lacking midrange warmth and fullness.
I get a similar vibe when comparing Primo 8 to an RE-600 that is not amplified. The RE-600 really need to be driven by the HM-901 balanced DAC/amp to shine and come close to the Primo-8. The W4 also wake up quite a bit with the HM-901, and sound very much like an amp'd RE-600 in that situation, but with the IPhone 5s or iPad Air headphone out the un-amped Primo 8 will excite me more.
A note on tips. I have been using Complys foam tips with the Primo 8, just like I do with Westone 3. I haven't tried other tips yet, but I do know the W3 frequency response can sound a lot more V-shaped with a vinyl tip, and more natural sounding with the Complys. I'll dig up some other tips for the Primo 8 to see how they handle that soon. Right now the Primo 8 and the Westone 3 complement each other perfectly when both are using the complies tips. I tend to prefer the W3 with older classic rock and the Primo 8 with jazz and acoustic, with a lot of overlap between the two with other genres.
Be back soon!