The TS9DX Turbo Tube Screamer was created in 1998 by John Lomas at Hoshino USA to meet Ibanez (Hoshino's) 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 of the original circuit.
The changes in the circuit for the 4 modes are :
Clipping diodes : there are several sets of diodes on new boards mounted on the control knobs, that are switched by the MODE knob.
Tone Capacitor : There are several capacitors on these new mini boards which are also switched by the MODE knob.
The four modes are described by Ibanez as :
TS9 : The classic TS9, no modifications. Like a TS9/808 or an old TS-808
+ : Grittier, louder than the original TS9.
HOT : Crunchier tone with boosted mids, like a wall of 4 x 12s. More volume and more lowend
TURBO : Powerful, bottom thick sound, like todays modern/alternative guitarists like. More volume and bass.