Stage 1a: Colour the lines uniquely
German whispers let us colour each line hi/lo.
The X in row 6 ties two lines' colours together - note that column 4 has 3 of one colour.
The X in row 5 ties the other two lines' colours together.
The X sum in column 4 causes a second cell of one colour to appear in a column with three of a colour, and thefore forbid them from being the same colour.

Arbitrarily, I'll call the colour of r4c4 "dark" and the other colour "light", with no prejudice about which is high or low.

Stage 1b: colour additional cells
The 5 in the centre cell lets us colour column 5 and row 5.
All the dark cells are marked in boxes 2 and 8.
Box 2's 5 and final light cell are ambiguously in column 6.
Box 8's 5 and final light cell are ambiguously in column 4.
All the light cells in box 6 are marked, therefore the final light cell in box 4 is ambiguosly in box 4 row 6.

Stage 2: Number/letter the cells
Given that the only rule that disambiguates high from low is the 3 in the corner, arbitrarily selecting light to be high and dark to be low, or the opposite is *not* a bifurcation. As soon as the corner reveals itself, you can flip all digits to their 10's complement instantly, and continue - it's not trying one thing, failing, backtracking, and trying an alternative. Purists may instead wish to solve using ABCD and FGHI instead. I'll use the latter, and arbitrarily put ABCD in dark cells, and FGHI in light ones.

Basically - centre-mark everything that's not both light and dark, that's the simplest way. Sure, some cells will end up with ABCD5 or 5DEFG markings, but it's not too noisy.

Stage 3: Sudoku
No great insights required for the initial waves propagating along German Whispers or across X sums, apart from one *essential* insight:

Essential insight: In box 5, the cells on german whispers may only pair 1-6, 2-7, 3-8, and 4-9.

The first wave starts with box 2 and column 4, there's a light GHI triplet, which restricts Fs also in box 8.
The changes that makes to box 5, with the X sums in r5c7, force that to be a 2 with 8s on either side. Then follow the waves. 
Because of the essential insight, almost every time you change markup on one cell in box 5, you'll alter its partner too - that is the trigger for most of the waves.

Working out the essential insight is the only thing that makes this a 2/5 difficulty - if you do waste time without it, when you finally work it out you'll consider it an "aha" moment.