Originally Posted by PurpleAngel
The output impedance is not a factor for driving the headphones, voltage (and current) seem to be what counts for driving headphones.
I would guess (?) your MOTU can decently drive the 250-Ohm headphones.
Actually, output impedance is
a factor in driving headphones, as the relationship between amplifier impedance and headphones impedance creates a voltage divider, limiting the efficiency of getting power into your headphones. If both are matched, then the voltage as measured at the headphone jack will be half of what the amp is trying to output.
Of course, power is what we're after, and power is a function of voltage and current, so very high headphone impedance can limit how much power the amp can supply. If you want a good image of this, check out the "specs" tab on the Valhalla
(30 Ohms output impedance). The Max Power specs are:
100mW RMS/channel @ 50 Ohms
420mW RMS/channel @ 300 Ohms
210mW RMS/channel @ 600 Ohms
You can see that amp can deliver the most power somewhere in the middle because higher headphone impedance will limit the current, but lower headphone impedance will limit the voltage.
Anyway, not to get too off track...the biggest component to "will this amp work" is probably going to be your headphone efficiency. The higher the efficiency, the easier to drive. From there, if you have a high impedance (and 250 Ohms is kind of high) you can also check maximum power output of your amp @ that impedance.
I would look these things up and list them, but IE is acting up and I have to dash. I'll try to get back to this later.