I really love the tuning on the two Acoustune IEMs I've tried and now own. They don't isolate well. They're not the cleanest most revealing IEMs around, but something about them sounds so right to my ears. I can just enjoy the music without really having any gripes. Nice and smooth sound. Since I only received the HS1650 CU today and did not bring my HS1551 CU to work I don't want to offer too much in the way of comparisons at this point.. I may have a better sense of things in a few days after directly comparing the two, but the general sense I have is the 1650 is a little cleaner and tighter sounding. Slightly less thick. A little less upper mids and a little more treble. Both are highly enjoyable and definitely not world's apart in performance. I think it's unlikely that the HS1650 is worth the extra money as it's about 420USD vs 660USD from AccessoryJack if you get EMS shipping. The only reason I gave the 1650 a chance is because I wanted an IEM to keep with my home setup while my other two IEMs are used on the go.