Impressions are in the order I tried them, while listening to Shostakovich's Symphony no. 8, 3rd mvmt on the Delos CD "Engineer's Choice II".
The Arcturus was a bit of an improvement over the stock JJ, but not really enough to warrant buying it.
The Tung-Sol and my Apheared 42 sound somewhat alike, although the Tung's treble is smoother by a bit. I really like how the Head sounds with this tube.
The Sylvania 12AU7's treble is a bit recessed, but the bass is nice and meaty. Yet after a few minutes, it's starting to sound slightly muffled?!? Go figure.
Unlike the 12AU7, the Sylvania 5751 has nice, crisp treble and nice, deep bass. It does very well with ambience, and the midrange is deft and sounds like a steak with excellent and slightly sharp sauce. Listening to Mussorgsky's Dream of the peasant Griotzko
the treble is REALLY nice and clear. This tube is good. How much do you want for it?
Okay, now that I've done about an hour and 40 mins worth of listening, I'm gonna go do some homework. But stay tuned, there's more to come, from The Wonderful World Of MGHeadDT(OTL) Tube Rolling as Made Possible by our sponsor, Nick Dangerous, and the Head-Fi Message Boards.