Practically any amp or preamp will hiss if you don't give it a hot enough signal prior to entering the channel strip or preamp input. Raising the gain on your mixer only increases the hiss even more and the mixer's circuits add their own hiss to the mix. The secret is to let the sending unit, be it a musical instrument or other audio signal, have a high enough level that you don't have to adjust the trim knob or channel fader. In fact, it is better to bring the levels down rather than up. This is partially a principle of gain staging. Of course, you must also avoid distortion with too much level input. The use of balanced cables, as well as +4 dB levels, also help with proper levels and hiss reduction, but if you are running a mix of -10 dBV and +4 dBu equipment, it's better to choose one standard and use line level converters.
Here's an article from Electronic Musician on how to DIY your own line level converter.
Hazel, I also noticed the hiss with my Studio V3. I know there are some mods to make it better, specifically replacing the stock tube and some opamps. But using the gain staging principles also helps.
