It is, yes, and I agree, I wish you could switch it off. I'm not sure it's a discrete part of the circuit that can be switched on and off. I wonder if the Tech21 Landmark amp head has speaker sim on the speaker outputs or just the XLR out?
Anyway, other preamp boxes I want to try out are the Hartke Bass Attack, Tronographic Rusty Box, and Aguilar Tone Hammer. I'm interested in the Rusty Box as it's supposed to be an agressive sound, and Dangerboy on this forum is a fan of it and it made him sell his Sansamp. The Aguilar Tone Hammer is brand new and looks promising too...