I've owned a pair of Heresy III "Capitol Records 75th Anniversary" edition (gee) for a few years and after the piles of great reviews of the IVs I bought a pair as, hey, they're "improved." I'm a long time musician, pro sound engineer, audio geek, jazz fan, and certified old person with Golden Ears, or actually flesh colored but experienced earballs. Or at least I know what I like. In any case the IVs are built better than the IIIs, new designs for the mids and ported, etc., and I was more or less happy with all that and hopeful during a 2 month break-in period. The issue became a pronounced upper mid, ear disturbing "blat" that I hoped would go away....most noticeable when a piano reached certain upper range notes that suddenly would jump out and harsh my mellow. My IIIs don't do that...they're pretty much flat and clean all the way up, and the IIIs have a much more complicated horn loading "phase plug" design far different than the poly driver bolted directly to the horn on the IVs. A VERY different mid driver design with the IIIs that simply works. Plus the IIIs are titanium mid diaphragms...also making 'em different. I use a couple of REL subs so the extra bass (not that much really) from the ported IVs was irrelevant although sounded similar to the excellent bass of the IIIs. Suffice to say the IVs went off to a new home...I did wonder if the annoying tone issue was specific to that particular pair, but the whole experience just made me happier with the IIIs which sound fabulous.