Hey Beat,
I've got a Theta Basic IIIa DAC I've been real happy with for 3-4 yrs.
It's been a trooper. Sure, I've heard a few other dacs, but none got me worked up enough to move up or over.
The one bite about Theta though is that they are ruthless on their upgrade/repair pricing. Not much on warranty on repairs as well. (3 months on the Theta Basic laserdisk I had fixed {for $400!) and it lasted 4 months.
The IIIa was well reviewed in Stereopile, way back 'in the day' and that, at the time, is what I bought by. It is one piece of hardware I've had little room to regret.
If you find a bargain on a IIIa or better, you might like it.
Good luck!
Regards,
DeadFish