Well, I guess I can take a stab at answering this (please excuse the really LONG post) ? as I have owned both at the same time and was able to test both at the same time side by side ? Hope your still digging the M4 (as I know I did). Along with testing many of the Randall mods I was able to test both single and dual channel mods (for example I have the single SL, and SL++ mods, and I had the dual SL and SL2 mods). Much of the difference (at least in the head?s ? RM100 vs MOD100) is the transformer ? and more than likely hand tested and picked components (in both the pre and head?s), which means much tighter tolerances of the parts ? I guess you would say higher quality. The MOD100 used a Heyborer (sp?) transformer and the RM100 uses a very close knock off. I also went so far as to match the tubes that were in the heads (Tung Sol 12AX7?s and a pair of JJ E34L?s with a pair of JJ 6L6GC?s). I also matched the pre tubes (though the M4 only had two ? as it had no effects loop) ? mated with a VHT 2/90/2 with Tung Sol Reissue 6550?s. The major difference seemed to be in the smoother high end (more refined?) of the MOD100 ? as such the Egnater seemed darker (hence less ?aggressive?). The differences also seemed to be in perceived gain (i.e. the dual SL mod) on the A channel was close to the Egnater SL single mod (but the single had more gain) ? and was again a little smoother with the dual. The B channel of the Dual SL had more gain (close to the single SL) but was a bit thicker in the upper mids (though the single SL has a ton of range and is really good) again the difference was in the high end ? but I could get a ?heavier? tone out of the single as compared to the dual ? I guess you would say scooped Marshall tone. Another difference seemed to be that the Egnater wanted its master up higher to get to where they sounded good, where as the Randall sounds better with the mods up higher and the master down lower ? but this only holds true for the Pre?s (I ran the Egnater at between noon and 1 on the master ? and the randall I run between 10 and 11 o?clock). My other guitarist really liked the Egnater sounds ? as he really likes the smooth 80?s style tones (Def Leppard Hysteria type tones) and I am more of an SLO/early Recto/JCM900 type tones (a little more gain ? ?aggressive? or ?crisper? high end ? a little more modern) ? stuff that cuts through. I should also not that prior to now I was in heavier types of bands (Sepultura, Pantera, Antrhax, Testament ? and the lighter side was Skid Row, Motley Crue type hair metal). Again their all great, but I guess it boils down to taste ? smoother, less gain, darker or brighter, more gain, more aggressive. I will never play another amp again I really, really liked the Egnater, but for me I love the Randall ? plus with the ability to have the modules modded (I feel like the Randalls can and sometimes should be modded ? but the Egnaters should be left alone ? as Bruce really knows what he is doing and the tones are great) ? leaves the possibilities limitless with this system.