Below the tonal balance differences between HE-400SE and FT1 pro
comfort-wise the FT1 pro (my opinion on my head) the FT1 pro is the winner here.
The tonal balance of the FT1 pro is also better and is lacking the 'etched' treble and has a bit more clarity than HE400SE.
Below the Sundara versus the FT1 Pro
The FT1 pro is far more comparable to the Sundara with the edge going to FT1 pro.
Also in comfort these aren't that far apart nor price.
FT1 pro has less clamping force and weighs as good as the same.
The FT1 pro will play louder at the same volume control setting as the Sundara.
So... having a FT1 pro next to a HE400SE is like owning the Sundara next to HE400SE. You would probably end up not using the HE400SE as it is more of a side-grade than a true upgrade. A small step.
No original pads are available for the FiiO though so only aftermarket pads (that may change the sound) is what one would end up with after 1 or 2 years of regular usage.
Below the HD560S versus the FT1 pro
Instead of the FT1 pro you could consider the HD560S, the HD550 or HD505 instead of the HD6XX(HD650)
When the goal is different comfort (low weight, very low clamping force, low price), a bit more deep lows and the sparkly treble of the HE-400SE you should consider the
ATH-R50x as well. Below the ATH-R50x compared to the HD650 (=HD6XX with different cable and color).
Clamping force of the HD650 (out of the box) is very high, the AT is very low. Both headphones can have the clamping force adjusted (takes careful bending at specific points)