I have the dt-990 (600) and was decided after I compared with the following:
beyer T50P (32ohm),
dt-880 (both 250 & 600),
dt-990 (250) and
senn HD 600.
600ohm does produce better sound. smoother high and deeper bass. It sounds more in control. It is quieter, more stable, therefore less noise, hiss, hum and static problems. Also I was advised by beyer agent in Singapore where I am located that 600ohm would sound better. between dt-800 and dt-900 would be personal preference. I personally prefer dt-900 sound. It is more exciting compared to the dt-880. Didn't like dt-880 (250) but the 600 version was indeed better.
To drive a 600 ohm headphone, I think a tube amp would be a ideal match. However, not any tube amp. A OTL tube amp would provide more power to drive the 600 ohm phones compared to one that is transformer coupled.
A OTL tube amp is ideal to drive a 600 ohm phone because it can provide the most power or voltage to drive it.
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I am driving my dt-990 (600) with a LD Mk IV SE. Easy to drive, beautiful sound.
Enjoy!