I've read a couple of reviews of universal demo units of customs that aren't normally sold as universals. There are some very minor differences in sound I've heard, can't remember what they are though. As long as you have a good seal, you should be close enough.
I wouldn't talk too much about fit if I were you.
Yea I definitely wont be talkin about that haha since I have no idea how the fit is. Generally you'll get slightly better bass and treble extension with customs. I also find that the midrange tends to be a bit more forward.
I did some comparison and writing on the Ei.3 compared to the earwerkz Supras. I guess here's a sneak peak of my review in progress for those interested:
CustomArt Ei.3 and Earwerkz Supra 2 (CIEM)
I consider the supra 2 to be the king of value as the supra is fantastically detailed and natural sounding for a 430 dollar CIEM (or 390 for the universals). So being about 100 dollars less, how do the two compare?
In terms of sound signature, the supra and the Ei.3 are more similar than they are different. However, the way in which the music is presented is quite different. For me personally, the past year or so has been fantastic, as more and more companies seem to be nailing the sound signature which I consider to be “natural” – which is generally s very slight V-shape with just a tad of extra energy in the upper mids. I would consider both of these IEMs to be close to my ideal sound signature. This sort of signature can often come forth as being fairly dry or boring, but the Ei.3, in particular, is a very musical sounding IEM.
Directly comparing the supra and Ei.3, the supra has a punchier and lower extending bass, a more aggressive upper range, and an overall more forward sound. The overall sound of the supra are crisper but dryer with better detail and overall texture as the presentation of the music from the supra are fairly analytical. Compared to the supra, the Ei.3 can feel as if it has a softer attack on the music as it’s not as sharp sounding as the supra (not to say they’re harsh sounding). While the supra are more detailed than the Ei.3, I found the more musical and smooth sound of the Ei.3 to actually be preferred over the supra’s sound. The Ei.3 also has the benefit of having a better soundstage than the supra, particularly in terms of depth and width. The supra’s one big weakness to me is its very underwhelming soundstage.
In summary, I find those who enjoy a more analytical and dry sound will like the supra, while those who enjoy a smoother, more musical, but slightly less textured and detailed sound will find the Ei.3 much more appealing.