My set up is a JMP-1 as pre-amp, into a Marshall 9200, into a 4x12 Cabinet w/ G12 V30 - Celestions. One side for the Wet (G-Major 2 TC) and the other for the Dry sound. I usually play a Gibson Axe, eventually a Fender US - Floyd Rose Series H-S-S. My music is Hard Rock, eventually into Metal zone.
The 5881 died on me on one side so I have ordered this kit as a replacement, after a long time spent on forums gathering more info. Now I can run A B testing as I have both kit's installed (5881 on A and 6L6WGC on B). The 6L6 provides a deeper bottom end and more highs when compared to the 5881. Bear in mind that the 5881 are older, so that in itself has an effect on the sound.
What I can attest though, is that the 6L6WGC produce a very tight and focused sound with immense attack. That's the Marshall tone of the 80's I have always loved. So right now I have the Dry sound on the 6L6 and the Wet on the 5881. I will definitely buy another Kit when the 5881 run out of juice.
Quality wise they are very well built, sturdy construction, and from what I have measured hold well the voltage on the plate, as the 5881 would.