Sorry, I meant the 1/4" DI output jack on the back of the 400RB. Do I run a 1/4" to XLR cable from therinto the desk or a 1/4" to 1/4".
If it's unbalanced then there's no point using a TRS to XLR cable. Best option is to run a normal guitar lead (a very short pedal board patch lead ideally) into a simple passive DI box. There are options from £10 to £1000. I like the Radial SB2.
Second best option is to do the other thing the manual says. If your mixer has a high impedance input (often the first two channels) then just use a normal guitar cable to plug it in that way.
DI box option is best. No doubt. You'll get away with the guitar cable method 9 times out of 10 but when you hit a room with dodgy electrical wiring you'll be grateful for the DI box.
Oh, one more thing. If you're using the guitar cable method please be very,, very careful not to accidentally plug the 1/4" speaker outputs into the mixing desk. Electrocution, fire, death and general debauchery will ensue. Don't blow up a soundman's desk, they have pocket knives, cable ties and gaffa tape.
Situations like this are why modern amps will have speakon outputs for the speakers and xlr outputs for the DI. That way you physically can't reach behind your amp on a dark stage and accidentally plug an amp that is providing a kilowatt into a desk that is expecting a less than a milliwatt.
Is the out post preamp? .... Post boost?