Hissing sound coming from tweeters

Try turning off everything but the amps and see if you still have it; if you do it is the amps. If not turn on the other components in order and see when you get it.

It's the amps for sure, I just don't know why they're making my tweeters hiss, and I want to eliminate it. The hiss comes on and stays on when I power up the mono blocks. The pre-amp never actually turns off. Do you think the mono blocks are a bad match for the rest of my system, or can I fix this problem with some sort of line conditioner?

To verify its really the amps, you need to disconnect the interconnect connected to the amps (do so with the amps off of course). If the 'hiss' is still there, then you are hearing the noise floor of the amp's circuit.
Would a conditioner reduce this noise? Maybe, depends on the conditioner.
COuld also be a mismatch between speaker and amps...speakers being too sensitive for your amps.

What you may well be hearing is the amps' "noise floor." The higher the sensitivity of the speakers, the easier it is to hear it. Every component has a noise floor. Not that I'm an electrical engineer by any stretch of the imagination, but I think it's impossible to not have one. Some are quite lower than others, and some may be inaudible.

It could be something else, or a combination of several things. In my experience, hiss is usually amp noise floor. Humming and buzzing are usually something else - ground loop, EMI, RFI, dirty power, etc.

The speaker sensitivity is 101dB so you are going to need an amp with a very low noise floor to not get some hiss. As Joselm said, disconnect everything to determine if it is the amp noise floor or not. The amp certainly looks like it could be the weak link. If possible, borrow another amp to try. You probably do not need a lot of power for those speakers. The UPA-1 is 200 watts and has a gain of 32dB, which is a lot for 101 dB speakers. It is the entry level Emotiva which may just not be quiet enough for your very sensitive speakers.