Its funny, I decide to check in on the forums, and there is a question I'm actually qualified to answer lol. Doesn't happen every day.
Anyways, I found the hiss to be about the same as the poor quality amps I've used (and some portable devices, AV receivers). It all depends on your tolerance for it. I find that most of the time, the music covers it up completely without issue. If I'm listening to a classical or acoustic piece that gets quiet, then it becomes noticeable, but honestly I find it to be part of its charm.
As for the sound difference, very few things in the audio world result in huge differences, especially once you reach these types of products. Compared to what I was using before, I did find the difference noticeable, and in a good way. It removed the sterile edge while still keeping that top end sparkled when its needed, added "warmth," and made the whole experience much cozier.
In my opinion Vali is one of the best amps out there for the money. But then again, I don't know your sound preferences.