Interesting post by Santiago over at Marshall:
Lots of speculation... The only I can clarify is that I started those amps and they were pretty much complete when I left BUT the amps that are released are different so I don't know what's actually in there although obviously the platform is the same.
As per what the amps were by the time I left: full tube amps, somehow related in a way to a 2061 with switchable extra gain (no extra stage nor solid started booster), Bass, Mid, Treble EQ and Tilt control. They had reverb, FX loop, emulated line out, footswitch and a 4 position attenuator so depending on the front panel power setting one could fine tune the final level with the attenuator or record without completely in silence by taking the signal after the power amp using the internal attenuator as power soak.
They were 50W/5W and 18W/2W 2xEL34 push-pull plus 5W/1W 1xEL34 switchable on the front panel. I had a combination of class AB/class A and cathode bias/fixed bias with different HT voltages for the push pull amps so the class of operation and output power could be changed depending on the front panel setting. The 5W single ended was the same amp as in the SL5, which is basically half standard push-pull running in class A. Whatever is in the current amps, I do not know.
The amps were started as the crunch channel for a Satriani combo and the tilt control is an idea to try to balance the brightness that many amps have when the gain setting is low. I presented the function as something similar in a way to be able to balance the treble and normal inputs in a 4 input amp although is not 100% the same. The name Tilt comes from a relatively old EQ method that with of single knob either boosted the bass or the treble by 'tilting' the frequency response from \ to / if that makes sense.
The original protos are featured in the song San Francisco Blue by Joe Satriani in both rhythm and solo tracks, appeared pictured in some magazines on top of several vintage amps and also appears in few videos of Paul Gilbert as he really liked the amp when I presented it to him so we sent a prototype for him to play and send feedback for development purposes.
Regarding the final released amps you'll need to wait till someone checks the amps or Marshall discloses more info... I can only talk about what I had in my bench by then. Obviously the reverb, line out and attenuator for example are gone, perhaps some other stuff has been changed too so take all the functions I tried to explain with a pinch of salt.
In any case I've heard the new amps and they sound very good to me.
Hope it helps!