Jeff uses several preamp modules from Salvation Mods in his 2 Randall MTS units.
[site | modules]

Baseman

Emulates a 1959 Fender Bassman, used for Jeff's main clean tone. Appears to have replaced (one of?) the stock Randall MTS Tweed modules in Jeff's rig in late 2011.

Duoplex

Emulates a Marshall 1959 Plexi. Appears to have replaced the Mash-All module in Jeff's rig in late 2011.

Mandarine Stonerverb

Based on Orange Rockerverb, this is Jeff's "main distortion sound." Jeff's is the special STARZ version, w/ two extra 3-position toggle switches.

Mash-All+

Emulates 4 classic Marshall amps: 1959 Plexi, JCM 800, Jose Modded, and JVM

Salvado Deluxe

Based on Salvado SLO overdrive w/ unique "bite" switch. Used by Jeff for leads.

According to Salvation Mods founder Antonin Salva, Jeff also owns the Graphic OR, Loneclean, Stoner AD, Twinface, and Voxy Face modules, and apparently Billy has asked for some custom modules as well.

Jeff said the following about his amp rig in October 2011 (article):

I'm still using the Randall MTS modular system but now I'm using a set of custom modules made by Antonin Salva from the Czech Republic. The man is a total amp wizard. His company is called Salvation Mods (www.salvationmods.com). My Randall preamp has four channels:
Channel 1: Randall Tweed (this is the only standard module I'm using);
Channel 2: Salvation Mash-All (emulates four different Marshall amps- 59 Plexi, JCM 800, Jose Modded Marshall, and the JVM series);
Channel 3: Prototype of a custom module Antonin is making for me based on an Orange Rockerverb head. This is my main distortion sound. It sounds amazing;
Channel 4: Salvation Salvado. Module based on a Soldano head that I only use for leads.