Tried these 'Iron Man Inspired' Acoustune HS1501 AL / HS1551 CU a while ago in Zeppelin Singapore. Couldn't find the dedicated thread for those models, so I start one to share impressions. http://acoustune.com/project/hs1501hs1551/ They are both have the smooth warm and bassy signature, but the Red-Gold one HS1551 CU is less bassy and more balanced than the Blue-Silver HS1501. The Blue HS1501 bass a bit too much for me and sometime sounds a bit bloated. Treble-head can pass these two models, they are definitely more suitable for those looking for smooth warm and full bodied sound signature. For those who like realistic bass power and dynamic, like me, the Red-Gold HS1551 CU may sound relatively balanced. Still rather bassy but in a good and musical way. Bass is strong and punchy, mids is smooth and warm, treble is super smooth, good clarity with decent extension, but treble level is a bit under the bass and mids. Dynamic is really good, a lots of oomph and realistic dynamic. I prefer the Acoustune Red-Gold HS1551 CU more for pop, vocal, and jazz. EDM I think is also good with it. I prefer to have more treble and transparency for classical and instrumental, especially acoustic instrumental. Overall it is a very nice sounding IEM with the right type of recordings.