Just wanted to let you know that there is a way to get more power from the single-ended output and it is not super-intuitive.
Gains 1 & 2 allow a higher input voltage than gain level 3.
If you find a hot source of 4-7V, you can use Gain 2 and get a lot of throughput.
Here is some info so you can get maximum output with your 789:
Where we landed in Gains:
Balanced: Gain: 0.66x , 2.0x, 6.6x (-4, +6, +16 dB), selectable via front switch. Output impedance: < 0.1 ohms
Single-Ended: Gain: 0.33x , 1.0x, 3.3x (-10, 0, +10 dB), selectable via front switch. Output impedance: < 0.05 ohms
The 789 can tolerate the following rms input voltage from your DAC without clipping:
-10dB Mode: 7 V
0 dB Mode: 7 V
+10 dB Mode: 2.1 V
On the 789, the RCA unbal and XLR-3 bal have same input voltage tolerance.
Here are two good options for you that will avoid clipping at any volume level with a 4.5V DAC:
1) Use 789 in 0dB or -10dB mode all the time and you'll never clip even at full volume. You'll achieve 4.5V for your TRS headphones, and 9V for your XLR-4 balanced headphones.
2) To access the maximum voltage swing... attenuate 7dB in your player (e.g. Foobar) or DAC, and then use the 789 in +10dB mode. That'll achieve 7V at your TRS or 14V at your XLR-4, and never clip at any volume level.
You should avoid the max gain position unless your DAC is guaranteed (e.g. with attenuation) to not be putting out more than 2.1V. Else you'll clip the preamp inside the 789.
The -10dB setting is mainly for sensitive IEM/headphones, where lowest noise and distortion and vol knob usable range are the goals, not 1000's of mW of output power.
Here, the max output voltage/power are also reduced by 10dB.
If you can't get adequate SPL at -10dB, then switch the gain knob to the 0dB (or even the +10dB setting if you have <2.1V DAC).
You can safely change the gain knob -10/0/+10 realtime without any clicks/pops.
The 7V/+16dB output is actually unclipped. Clipping (1% THD) starts a bit higher than that.
XLR Input impedance is 50k differentially (line-line) and 1Meg each line to GND.
Power button: If you forget your amp is on, it is I think 6 hours with nothing coming through and then it will automatically shut itself off. That means we have to have a momentary switch that only serves to try and turn the unit on...so it returns to ready state each time after you press it.