Fantastic. Combined with the Playmate 2, it transforms a rather laid back warm HD650 into a beast. The opamps keep the lush and forward mids unchanged, but help tremendously with the bass and highs. Bass is accentuated and becomes more aggressive, more punchy, goes slightly lower even, but it’s never bloated. Highs are more pronounced without becoming too harsh. Overall the Vivid opamps manage to transform my HD650 into a less-detailed Focal Clears (without that dreadful canion in the upper mids on Focals lol). Fantastic sound, totally recommend, especially to any HD650 / HD6XX owner. I’m probably going to stick with these opamps for good, I’m totally satisfied with their sound.