I will say that of the 3 latest iem's I have owned the SM64 takes the most power to maximize the sq. The UM3X was the easiest to drive, W4R were in the middle and the most difficult definitely the SM64. I used to use an S3(Samsung) and clip zip to power my UM3X and as I purchased the W4R I also purchased a Fiio X3 dap to power them. With the W4R I would use low gain and 30% volume. I have found to reach the same level of sq I now use high gain and 30%. I prefer the sound signature of the SM64 the best.... But I noticed right off they took more to drive
exactly my experience with the sm64 and westone 4s. my dap is the dx50.