Hissing sound coming from tweeters

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.

To further elaborate on the test methods recommended when disconnecting the interconnects from the amp use shorting plugs at the amps inputs.

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What Dtc said

IMHO your amps are not an ideal match for your speakers. You have way too much gain and even though price does not always indicate quality you have a $3,500 preamp with $300 amplifiers. I think you need to pony up for an amp that is a better match; less power, less gain, higher quality.

To test it don't just power it up with the inputs disconnected, many amps will be noisy when run without inputs. You need some shorting plugs in the inputs then turn it on to see if it hisses by itself. Or disconnect the interconnects at the preamp and short the middle pin to the ring. Stuff a little aluminum foil in there or use a jumper with clips.

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