SVS is a respectable company and I would expect all of their products to work without any audible hiss from themselves. That said, it's more likely that the hiss comes from the source that you have plugged into the sub(s).I just bought a pair of the SB-3000 subs. I was really enjoying them until this morning. I used the remote app to turn on the left sub first. I heard the power click and then a hissing noise. I then turned the right sub on and experienced the same hissing noise. I was sitting a good 12 feet away. I am 61 years old and worked at a job that was not kind to my hearing. I got up closer to each sub and could clearly hear the noise. To me it almost had an oscillating sound to it. My listening room was quiet at the time, except for my Synology NAS. The NAS has one noisy hard drive.
I am torn as to whether to keep them or not. I can't think of any other subwoofer with an app that works so I can have them hooked up to a home theater preamp and an audio preamp. I can easily switch between the two from the app. I wonder if the SB-4000 has the issue. I do not really want to spend the extra money, but I should not hear the hiss from 12 feet away.
A few things to try: 1) does it hiss, if there is nothing connected to it, just the power cord, and switched on? 2) Does the hiss start when you connect other device to it, that is switched on? 3) Are the connected devices in the same power outlet, and if not, does plugging to one help?
I'm sure SVS support would love to help you on this if just bought a pair!