Line & Monitor Outputs

Dynamic Range Outputs (1-2)
106 dB (A-Weighted)
Dynamic Range Outputs (3-4)
103 dB (A-Weighted)
Maximum Output Level (0 dBFS) Balanced Line/TRS Outputs
+10 dBu
Maximum Output Level (0 dBFS) Unbalanced Line/RCA Outputs (1-4)
+5.5 dBu
THD+N Outputs (1-2)
<0.001% (minimum gain, -1dBFS input with 22Hz/22kHz bandpass filter)
THD+N Outputs (3-4)
<0.008% (minimum gain, -1dBFS input with 22Hz/22kHz bandpass filter)
Impedance (TRS Outputs)
94 Ω (Balanced)
Impedance (RCA Outputs)
47 Ω
 
With my Scarlet 6i6 the 3/4 are a clone of the 1/2. I use them to drive a small mixer I use for headphones. 
Make sure the 3/4 are checked in Sonars Preferences/Audio/Devices 

ptheisen
There is another thing you might check regarding the hum issue. Balanced connections can be an improvement on long runs, say more than 25 feet, but if you're getting a lot of hum from short unbalanced connections, there might be another cause. If the grounding of the electrical circuits in your home is not consistent from one circuit to another, and your audio equipment is plugged into multiple circuits, that can definitely cause a significant hum. If that is the cause, plugging all connected audio equipment into the same circuit may alleviate some or all of the hum.

Yes. All the gear is plugged into an APC UPS, hichnis plugged into a single wall plate.

sock monkey

Line & Monitor Outputs

Dynamic Range Outputs (1-2)
106 dB (A-Weighted)
Dynamic Range Outputs (3-4)
103 dB (A-Weighted)
Maximum Output Level (0 dBFS) Balanced Line/TRS Outputs
+10 dBu
Maximum Output Level (0 dBFS) Unbalanced Line/RCA Outputs (1-4)
+5.5 dBu
THD+N Outputs (1-2)
<0.001% (minimum gain, -1dBFS input with 22Hz/22kHz bandpass filter)
THD+N Outputs (3-4)
<0.008% (minimum gain, -1dBFS input with 22Hz/22kHz bandpass filter)
Impedance (TRS Outputs)
94 Ω (Balanced)
Impedance (RCA Outputs)
47 Ω 
With my Scarlet 6i6 the 3/4 are a clone of the 1/2. I use them to drive a small mixer I use for headphones. 
Make sure the 3/4 are checked in Sonars Preferences/Audio/Devices 

Thanks. I’ll check on the Sonar preferences. In the 2i4, manual says the 1/2 RCA outs carry same signal as 1/2 quarter-inch outs. Doesn’t say anything about outputs 3/4, except that DJs can use it for a second turntable.

A 2nd turntable? Its an output. What use is feeding an output to a turntable?
 
At any rate back to the hum. What kind of lighting are you using in the room? Any dimmers? What happens if you use a ground lift?