Beyer Dynamic DT 770 Pro 32 ohm ?

also worth exploring: Sony 7510 24 ohm

Re "working it out":

Read this and this.

At same sensitivity ("The sensitivity rating is usually for 1 milliwatt power input to the phones and a corresponding sound pressure level (SPL) output (usually 102 to 106 dB SPL output for moderate to high sensitivity rated phones)." source):

• Power = volts times milliamps.
• At the same voltage (amp output stage) more ohms = fewer milliamps (Ohm's law)
• Thus more ohms (headphone coils+amp output stage) require more volts (amp output stage) to get the same milliamps to get the same loudness (same sensitivity, higher impedance)
• Fewer ohms = potentially more sound coloration (source). But headphones are coloured anyway
• (Sensitivity is likely determined by the magnets and the headphone membrane materials.)

Fewer ohms = higher current (same voltage), but remember the maximum current draw requirement of USB - <500mA. With Mbox Mini in question too high current is probably not an issue, but the max voltage available to the headphone amp is limited and the output stage likely has relatively high impedance. Re voltage USB offers 5V max (Firewire bus has 12+). So to up the loudness your choices are lower resistance (=higher current) or higher sensitivity.

There are headphones with 24ohm (Sony above) and even 16ohm resistance. I'd stay at 32 minimum.

Also it's worth exploring the idea that you may be asking for too much loudness.

Beyer Dynamic DT 770 Pro 32 ohm ?

also worth exploring: Sony 7510 24 ohm

Re "working it out":

Read this and this.

At same sensitivity ("The sensitivity rating is usually for 1 milliwatt power input to the phones and a corresponding sound pressure level (SPL) output (usually 102 to 106 dB SPL output for moderate to high sensitivity rated phones)." source):

• Power = volts times milliamps.
• At the same voltage (amp output stage) more ohms = fewer milliamps (Ohm's law)
• Thus more ohms (headphone coils+amp output stage) require more volts (amp output stage) to get the same milliamps to get the same loudness (same sensitivity, higher impedance)
• Fewer ohms = potentially more sound coloration (source). But headphones are coloured anyway
• (Sensitivity is likely determined by the magnets and the headphone membrane materials.)

Fewer ohms = higher current (same voltage), but remember the maximum current draw requirement of USB - <500mA. With Mbox Mini in question too high current is probably not an issue, but the max voltage available to the headphone amp is limited and the output stage likely has relatively high impedance. Re voltage USB offers 5V max (Firewire bus has 12+). 

So to up the loudness your choices are lower impedance (=higher current) or higher sensitivity.

There are headphones with 24ohm (Sony above) and even 16ohm resistance. I'd stay at 32 minimum.

Also it's worth exploring the idea that you may be asking for too much loudness.

Beyer Dynamic DT 770 Pro 32 ohm ?

Beyer Dynamic DT 770 Pro 32 ohm ?

also worth exploring: Sony 7510 24 ohm

Re "working it out":

Read this and this.

At same sensitivity ("The sensitivity rating is usually for 1 milliwatt power input to the phones and a corresponding sound pressure level (SPL) output (usually 102 to 106 dB SPL output for moderate to high sensitivity rated phones)." source):

• Power = volts times milliamps.
• At the same voltage (amp output stage) more ohms = fewer milliamps (Ohm's law)
• Thus more ohms (headphone coils+amp output stage) require more volts (amp output stage) to get the same milliamps to get the same loudness (same sensitivity, higher impedance)
• Fewer ohms = potentially more sound coloration (source). But headphones are coloured anyway
• (Sensitivity is likely determined by the magnets and the headphone membrane materials.)

Fewer ohms = higher current (same voltage), but remember the maximum current draw requirement of USB - <500mA. With Mbox Mini in question too high current is probably not an issue, but the max voltage available to the headphone amp is limited and the output stage likely has relatively high impedance. Re voltage USB offers 5V max (Firewire bus has 12+). So to up the loudness your choices are lower resistance (=higher current) or higher sensitivity.

There are headphones with 24ohm (Sony above) and even 16ohm resistance. I'd stay at 32 minimum.

Also it's worth exploring the idea that you may be asking for too much loudness.