There's also an easy way to go if you just need to record a DI along with the main Stereo Pair (and don't need to separate DI/Amp/FX (as above) With this, you don't have to make changes to presets (excepting the FXLoop - see AX8 manual page 79-80)
In the I/O: Output 2 Send mode: copy L>R, Echo input.
"Selecting “INPUT 1” copies the raw unprocessed INPUT 1 signal to OUTPUT 2, and is ideal for capturing a dry track for “reamping” without using the FX Loop block. Note that the echo feature will not function when an FX Loop block is present in a preset as the FX Loop has priority" AX8 manual page 79-80.
When reamping use IN 2 Left Only - page 79 of the manual. You will need an FXLoop for the ramping preset