1 platinum tube $40) 12BH7A RCA black plates used/tests like new smeared white print "D" getter 1950's (12/12.3ma)
Buy this tube, it has amazing bass pressence and weight and the top end is very smooth. You will love it, especially with your 600s. This tube presents the most natural sounding bass I have ever listened too. Its taller than the 5687s too so it looks cool in the rear socket. I also bought an RCA clear top 12AU7, it also sounds very good, but its different, its more open in the highs and the bass weight is less.
I agree the 12BH7 is a sweet tube. Unfortunately, you guys cant use the 6GU7 which is the 6.3 volt equivalent of the 12BH7 and sounds just like the 12BH7 .... you can pick these up for as little as $4 each. I use the RCA/ Sylvania and made in Japan Raytheon 6GU7s for gain in several of my Singlepower amps, including my old chassis Supras. I love this tube.
If you want a really good sounding 12AU7 equivalent try the CBS Hytron or Raytheon D getter /2 mica/ blackplate 5814 from the early 1950s.