From my experience, all of the 6SN7 tubes I've tried had hiss.They do, but you probably just had bad luck. We use 6SN7s in everything we make and no worries playing them on high sensitivity loudspeakers. My speakers at home are 97.5dB 1 watt/1 meter, and no hiss.
@prof I had the same experience when I tried to use an SUT instead of going direct-in on my phono setup. The SUT was certainly quiet, but I found myself not playing the stereo anymore. When I got the SUT out of the system, right away my interest came back.