you should really try the TADAC from Tube Audio Design. They have it selling for $699 with a 21 day trial. I've had the Benchmark, Tact, Paradisea, Monarchy etc... In my system and to my ears(and a couple friends) this is the best of the lot... expecially for the $$ I haven't even tried different 12au7 tubes yet 
just my opinion 
Tony
I'll second that!!! The TADAC is a fantastic sounding piece. I don't know how Paul pulls it off for the extremely reasonable price. Hey, who am I to argue?? It'll also be a lot of fun down the road when I try my hand at tube rolling.

I emailed Paul & asked him if the DAC can be left on 24/7. His response was, absolutely...he designed it with that in mind. Apparently the tubes are only in circuit when a music signal is received. You'll see a faint blue ring glowing around the bottom of the tubes. The second the music stops, the faint blue light stops. Being on 24/7 is not a concern for the tube life. I'm a very happy camper!!