With the DT880/600Ohm since they are semi-open they will have a lower sensitivity. The general rule is more airflow - more sensitive, lower airflow - less sensitive. Since the DT880 is partially closed this makes them less sensitive. Even though beyer gives you the same numbers as the 600Ohm DT990 (600Ohm, 96dB) the sensitivity is actually lower. When I owned both, the DT880 was not as loud as the DT990 at the same volume setting on my amps.
All of these have more than enough power for the headphone, so they should be the natural choices. They are OTL tube amps, and as such they can output a large voltage swing an can drive higher impedance loads with ease. I've heard the DT880/600Ohm on a few of these (CSP2+, WA3, Crack) and it sounded beautiful.
OTL stands for Output Transformer Less, which means there is no transformer in the signals path nor is the amp coupled to one for buffering. The resulting output impedance of the amp is quite high and can be anywhere from 60 Ohms to over 100 Ohms. OTL designs generaly have a high voltage swing and are more efficient at driving high impedance loads. The high impedance will give some headphones a fuller sound and also gives you notice not to use any low impedance loads.
So the sensitivity (efficiency) varies from model to model?
Why can't they just tell it like it is!!!
Or is the DT990 apparently louder than the DT880 because it is brighter and the 96 dB @ 1 mW figure is referenced to 1 kHz?
But based on your statement, I assume the DT770 is the quietest of the 3? Which would ruin my brightness theory!
I see what you are getting at with the OTLs output voltage swing, but to clarify:
OTLs generally have more than enough voltage to drive high impedance phones but, in general, may not have enough current to drive low impedance 'phones.
While the efficiency of an OTL will be higher when driving high Z (impedance) vs. low Z cans, a Solid State amp with a virtually zero output Z will be much more efficient than an OTL with any impedance 'phone. This due to 1. the high output impedance of the OTL and 2. the energy needed to energize the tube's heater filaments.
Personally I prefer using my DT880/600 ohm with an OTL. This is just my personal preference; my soild state Matrix M Stage has more than enough output voltage to drive my DT880/600 ohm.