This is Fried Reim from Lake People / Violectric.
V281 offers not only a "convenience" 4-pin XLR socket but a true balanced output.
There are 4 amps inside !! It is a BTL or push-pull arrangement.
The balanced headphone output should be measured in a differencial way and never related to ground.
Otherwise the inverting amp will be tied to ground - causing distortions.
Assuming the same input signals and the same load the balanced output is double as loud (+6 dB) as the SE outputs
and the amp will deliver about 4 times the power.
Also, each of the 4 amps has its own current limiter, so 2 amps will really deliver 4 times the power.
Concerning the output voltage swing HPA V281 is the most powerful headphone amp delivering up to 40 Veff into 600 ohms = 2700 mW !!
The technical data from the manual are no cheats but true:
RLoad (Ohm) 600 => Ua (dB) 34,4 = Ua (V) 40,2 => Pa (mW) 2700
RLoad (Ohm) 300 => Ua (dB) 32,8 = Ua (V) 33,8 => Pa (mW) 3800
RLoad (Ohm) 100 => Ua (dB) 29,7 = Ua (V) 23,7 => Pa (mW) 5600
RLoad (Ohm) 50 => Ua (dB) 25,4 = Ua (V) 14,5 => Pa (mW) 4200
RLoad (Ohm) 32 => Ua (dB) 21,7 = Ua (V) 9,5 => Pa (mW) 2800
RLoad (Ohm) 16 => Ua (dB) 16,0 = Ua (V) 4,9 => Pa (mW) 1500
The peak is at 100 Ohm load impedance, for lower loads the internal current protection will act.
The output voltage @600 Ohms may suffer a bit due to poor US AC supply !
The above data was taken with European 230 V AC.
The low frequeny roll-off is due to the low impedance input from the relay attenuator circuitry.
This is not "nice" but normally not to hear.
With the "standard" volume control the frequency range is ruler flat down to 5 Hz (-3dB).
About the loose knob.
The shaft from the potentiometer is made of steel while the massive knob is made from aluminum.
Unfortunately this mix of materials sometime causes a loose knob.
Normally a 1,5 mm allen key is supplied with the unit to fix the knob.
To come out of this trouble we could use a smaller knob or one with a plastic insert - but we don´t want to ;-)
Best regards, Fried Reim