+1 Brighter, more aggressive in the upper mids, more sizzly up top (finally got to try one of these out briefly... quite similar to the MB500 to my ear, with a bit more character and complexity). The Streamliner sounds much more like an all tube amp. Not really scooped, just fat and smooth and big and warm and creamy, with a very accurate 'all tube' top end (almost a bit spooky).
I describe the Streamliner as a great micro choice for those who dig the SA200, the DB750 or the Mpulse600. The Fusion is much more similar to the Mesa M6 and M9.... more solid state than classic tube sounding, big down low, super bright up top, with more of a 'touch of tube pre' versus the all out 'all tube' emulation that the Streamliner goes for. I personally prefer the 'drive' tone of the MB500 for a more gritty tone than the pushed tube tone of the MBFusion, but that is pure personal taste there. Both seem to be relatively similar to me when run clean, versus the total night and day difference between the Shuttle and Streamliner in the Genz line-up.
Just like many all tube amps, and the DB750 and Mpluse600, cab pairing is a bit more key for the Streamliner due to that 'all tube' low end vibe. The MBfusion is much more solid state, with a nice 'tube flavor', and would seem to work pretty well with most cabs, like the F1 or Shuttle or whatever.
All impressive heads. To the poster with the LMII, putting a Sansamp VT pedal in front of that will get you into the MBfusion tonal universe IMO i.e., brighter and grindier, if you don't want to do a full head swap.