I have enough 6SN7's to start my own shop :) I have Sylvania, RCA clear glass, RCA grey glass, Elektro-Harmonix Gold, Western Electric, and the cheap Chinese jobbies that came with the SLP-05.
The Western Electrics are the most dynamic - clean, quiet, and with a lot of slam. They can sound a little sterile and do not have as nice a tone as the RCA clear glass.
I also like the Sylvanias - a less romantic sound than the RCA (which can sound fat and slow with the wrong recordings), but my Sylvanias are noticably noisier than the other tubes.
Another tip for you - the biggest improvement can be gained from changing the input valves (the two valves on the far left). Do not bother changing the tubes on the far right as these are for the headphone stage.