Thanks for the reply. How much did this EQ cost you? And I've been trying to get my hands on a Mark IV to compare to the III I have but there is no Guitar Center in my area and even the one in Atlanta doesn't have one. I'm wondering if that fuzzy gainy sound is just a part of the III and if I would like the IV better but since I couldn't try both out I went with the "sounds like a IIC+!!!!!" option lol. One more question you said you did have a III how would you compare it to the IV?
I paid $55 plus $12 for shipping for the eq on ebay. There is a guy selling brand new in the box ones for $76 plus shipping here http://www.ebay.com/itm/Presonus-EQ3B-Parametric-EQ-NEW-/261170037449?pt=US_Signal_Processors_Rack_Effects&hash=item3ccef282c9
My III compared to the IV that is a loaded question. My IV is the sound I have been wanting to here since I was 14 years old. It is everything I thought I was getting in the Mark III. It is Incredibly ballsy, in your face, and extremely smooth at the same time. When you strum a chord you can literally hear every note. It's almost like the distortion sits on top of the clean channel instead of being combined which keeps it very clear and smooth. Very hard to get a bad sound out of the Mark IV.
Now the Mark III. I am very convinced that the previous owner damaged it. He had been trying to trace the circuit in order to clone it. He liked to buy amps clone their circuits and then flip them.
I am not trying to bad mouth this amp only point out my experience.
Mine never had the amount of gain on tap that the Mark IV does. It had a lot of fizzy ice picky qualities to it. The presence knob was not useable and supposedly had a mod done to it at the factory as well as the reverb mod.
I did the IIIC+ mod to it as described on the forums. It tamed some of the shrillness. I also replaced all of the electrolytic capacitors in the amp. This improved it some, definitely in the low bass area.
It always felt a little stiff to me. I thought maybe it was due to being wired in pentode? It was very raw. No compression. Not tight at all. It also didn't take pedals in the loop well.
Once I added the parametric eq things changed. I got the compression, tightness I was looking for. So I never explored further. I was close to the sound I was looking for. After a while, I decided I also wanted a clean channel. Very hard on the Mark III. To me it was a one trick pony. But it did that trick very well.
I have watched many youtube videos since then of Mark III's and can honestly say mine never sounded like those. Most of the videos have nothing but a guitar, cable, and the amp. Mine needed the eq and a boost pedal to sound like theirs did raw. I now suspect that maybe one of the vactrols or LDR's might have went bad and it never fully changed channels. Should have sent it to Mesa for service.
I grew tired of it and switched to Marshall's for a while but began missing the bass the Mesa had. So I bought the Mark IV. For me there is a night and day difference between those amps. Like I said there was probably something wrong in mine.
Please don't trade or sell yours without making sure it doesn't need service. Listen to the videos on youtube. See if yours compares to theirs. I have heard many red stripe mark III's on youtube that sound great! If my mark III had sounded like those videos I would never have sold it.
Also my Mark IV doesn't need the eq. it is plenty tight on its own. There is just some tweaking to a couple frequencies I like that makes it a little nicer for me. It just tailors the sound.