I have a 900 SL-X with 5881...killed any GAS in an 800 for me...modded Marshall is covered by my Engl Artist Edition and Bogner Helios Eclipse....
As far as combos, I really like my DSL40CR for higher gain Marshall tones; worth checking out and a lot less $ vs an 800.
These days, I prefer my vintage sounding Marshalls (1959HW, JTM 45 reissue, Origin 50C) just ordered a Celestion G12M-65 creamback for the Origin, and a PRS Archon 1x12 with a V-Type 70W. With the power scaling and switchable effects loop, it's becoming the amp I use the most.