Such a complex method is only necessary if you want to use FX send from your amp to create a more complex signal circuit that also includes your amp's pre-amp.

To simply use the X3 like any old stompbox that you'd plug into your amp, without worrying about that – which it sounds to me might be what you want – is very easy:

Just go into the Outputs settings and change the mode to one of the Amp In settings. This disables cabinet modelling – which may well be responsible for the bad sound you reported, as it's not a good idea to route one cabinet into another! Of course, make sure you also turn off amp modelling in your customised tones.

It's that simple!