Not having that be user adjustable would be a disappointment, to me.
The control layout would seem to indicate a lot of adjustablility...
+1 As Red Planet mentions, it is a tube pre, so you will have the tubes in the circuit all the time. However, with the very nice pre gain, post gain and master controls (like on all the other Shuttle tube pre's), you should have the ability to go from VERY clean (I hope) to full out snarl while easily controlling both the absolute volume, and just as importantly, the level of signal hitting the EQ and the rest of the preamp circuitry.
And, more similar to the Max heads than the Shuttle heads, I assume they have the input gain set a bit hotter than the Shuttle heads (which have no input pads), and allow the user to 'pad down' the input signal if needed.
As has been discussed, one of the things about the Shuttle amps (the non-max versions) is that the input gain level is set pretty low, and if you have a relatively soft touch and/or low output instrument, it is hard to really push that tube into even minimal break-up. That should not be the case with the Streamliner.
And again

, regarding demo'ing the full tube break-up snarl of this amp, my hesitance is not on the mechanics of the knobs, but rather my playing approach.... it's like asking a guy to demo a hi fi slap sound who plays hard rock with a PBass and a pick. Don't get me wrong, I'll dial that tweeter down and crank that pre when I get it, but I look forward to having a player that really lives that tone and style to really represent that part of this amp in a demo.