I have experience of the Musical Fidelity X-Can v3 and the Sugden HeadMaster, with both the Sennheiser HD650 and the Grado RS-1 headphones.
In my opinion, the Sennheisers sounded superb with the X-Cans smooth and detailed with no hint of harshness. The Grados fared less well, with an altogether brighter, more fatiguing sound.
With the HeadMaster, the reverse was the case and the Grados were at their best with the Sugden amp. In comparison, the Sennheisers sounded too rich and lacked sparkle.
Build quality is good with either headphone amp, but Musical Fidelity holds the aces because the addition of their X-PSU raises the quality of the X-Can v3 to an altogether higher level and the combined cost of the X-Can and X-PSU is considerably less than the HeadMaster alone.
So I'd say you are paying a particularly high price for the remote control facility offered by the Sugden. Isn't there a way of routing the X-Can v3 signals through the X-150 so that you can use the X-150 remote volume control when using the headphones?