This might have already came up before, but thought I would address this as I reckon it makes my 275 a way more versatile amp. The FX loop in these tube preamp, stereo solid state power amps is parallel. Meaning it splits the preamp output and sends some to the power amp and some to the FX unit your using. This is great if you want to use Reverb or Delay etc, not so great if you want to stick a Boss Multi-FX unit in the FX loop for example. Do you really want your chorus or pitch shift to only be half there when it gets mixed with the preamp signal before heading into the power amp? Not me. Really don't see the point of a parallel FX loop on a guitar amp. I wanted a serial FX loop so I could decide on how much of an FX/signal ratio I want. Don't even get me started on the 4 Cable method. So anyway I asked Marshall to send me the amps schematics, which they did, (didn't think they actually would but they are cool guys). Me and a friend of mine who is electrically more clued up than me took a look. We figured Resistors R91 and R92 could be disengaged to completely separate the preamp from the power amp. Its a pretty easy mod, I cut out the two 15Kohm resistors and put a switch on the back panel, I ran shielded audio cable to the switch from the board resistor points and hooked this up to the switch (you put the two resistors back in at the switch end). The resistors are located behind the front panel. You have to take all the knobs off, pop the board off its tags and carefully pull it out of from the insides of your amp. I didn't have to disconnect any other cables or plugs to get it into a position where I could work on it. Solder your two pairs of wires into the resistor contact points and put a switch on the back panel. There are looms of cables you can cable tie these new wires to and then just stuff it all back together. Be careful! plus stay away from any thing that looks like big capacitors. I'm no electronics expert but if you know what a soldering iron is and can figure out how a switch works then your sweet. Now you have a serial FX loop, or you can flick the switch and have it back to its filthy parallel state. One thing you should note is that in Serial mode, you need the FX loop Level turn all the way up and the preamp out has to be "physically" connected to the power amp either by an FX Pedal in the loop or just a short patch cable. One little quirk is of this is the Digital onboard FX are still conected to the power amp at all times. This means you can select the serial mode on the newly installed switch and have your preamp disconnected to the power amp, play your guitar, and just have the completely wet FX sound of the on board FX's playing. Kind of cool, but Ive never used it other than just mucking around. Anyway, this really helped me out, now I use a Digitech RP1000 in a 5 cable method (Stereo FX RTN), and the whole thing works really great, I can also unplug everything flick the switch and its back to being stock. Oh, and its stereo, only thing i wish is that Marshall would make a simple (no modes) four channel stereo amp. Then I could have power amp valve tones. I know i could go rack stuff but I just want a half stack that stereo and has four simple "as me" channels. Maybe somthing like a DSL with selectable modes and a master volume section. Come on marshal, I'll buy one.