Here are a few good reasons why an IEC socket is a good thing...
LOOK AT THAT! The mains chord has actually pulled out of the grommet that is meant to hold it in place. The grommet no longer actually holds the cable steady enough at all. The strain of anything pulling on the chord is being taken by the LIVE wire soldered to the fuse holder, and the SAFETY EARTH soldered to the chassis.
The yellow/green wire (Earth) should be longer than the Live and Neutral wires so that in the event of any cores of the mains cable breaking inside the chassis (Say for example some Goomba comes and yanks that cable so hard it breaks at the solder connection, probably at the live wire) the safety earth will always be the last thing attached or at least be the last core to break off.
The other issue I have with the 6505 at least... is this plug adapter.
It's HUGE and you HAVE to use it because under there is a 2 Pin European style plug with no fuse, that you can't remove from the amp because the mains cable is fixed. This makes it really annoying to carry your amp around and even if you use a flight case, you always have this brick attached to your amp.
Most techs are probably gonna charge you a bunch of doubloons to fit an IEC but it's a quickish job and will save you a lot of hassle and potential death.
Step 1: Get rid of the grommet and cut the mains lead leaving enough wire on the fuse holder, voltage switch and earth to go back to the IEC. (Not really major surgery!)
Step 2: Best way to make a hole for the IEC is with a sheet metal punch. These are expensive, but if you can borrow one (like me) then its cool. A tech should probably have one for doing stuff like this. I removed the fuse holder because it was in the way simply by unsoldering the wire going to the mains switch from the back of it. You line up the cutting side of the punch and then assemble it again, and use a wrench to tighten that big nut which in turn pulls the cutting part through the metal. BEAST!
Step 3: Get your IEC socket, mark the holes for mounting it, drill em out.
Step 4: Install the IEC and wire it up making sure you get the wires back on the correct pins! This IEC would have stuck out a tiny bit at the top and bottom of the chassis where the metal curves around. For this reason it was mounted at a slight angle so it could be fixed in a solid fashion with no bits sticking out to snag on anything.
And thats it! Throw that massive old mains plug away and get a nice kettle lead. Stop worrying about the Live wire breaking because a Goomba yanked on it. Don't let a tech tell you that doing this is a major operation. Actually... to be fair... if you try and do this with a drill and a file instead of a punch, it IS a major operation and you'll get metal filings in your amp... so screw that!