S.E.X. will make 2 watts, which is 2.83volts rms when wired for 4 ohms.

Some headphones are rated in dB at 1 milliwatt (mW), others are rated in dB at 1.0 volts.

Power in watts equals voltage squared divided by resistance, so mW equals 1000*(2.83*2.83)/R, or 8000/R.

dB above the 1mW level is 10*log10(power in mW). Add that to the dB/mW to get max output.

So, assuming all four headphones are rated in dB/mW:

HE-6 @ 50 ohms is 160mW or 22dB above 1mW, giving 105.5dB maximum output

HE-560 @ 45 ohms is 178mW or 22.5dB above 1mW, giving 112dB maximum output

HD 650 @ 300 ohms is 25.7mW or 14dB above 1mW, giving 117dB maximum output

DT 250 @ 250 ohms is 32mW or 15dB above 1mW, giving 111dB maximum output