The TS9DX Turbo Tube Screamer was created in 1998 by to meet Ibanez marketing request for a tube screamer with different modes for output and distortion. It has an additional knob along with the old Drive, Tone, and Level knobs. The new knob is a 4 position MODE switch. The TS9DX has the EXACT same circuit as the original TS-9, but the mode switch changes some of the parameters of certain components.
The four modes are described by Ibanez as :
TS9 : The classic TS9, no modifications.
+ : Grittier than the original TS9.
HOT : Crunchier tone with boosted mids, like a wall of 4 x 12s.