Musical Fidelity X-10D

You would still be able to use the x10D in the same way and it would have a similar effect. You'll just need to do some listening and hear whether You like it in or out of the system

No, the X-10D is not a DAC. It's just a tube buffer stage. You could just attach it after the DAC and before the preamp. However, there usually is no need for such device unless you are using no preamp or a passive one. Some poorly designed CDP with low outputs might also benefit.

It depends-I used one in my system for a long time with an Audio Alchemy DDE3 and always felt you could hear a little warmth in the sound.
I was felt what it added was missing when I took it out.
I'm not familar with your equipment buy my opinion is that up to a certain level of equipment it will make a difference.
Having heard the MSB which to be honest I didn't rate much higher than the Alchemy stuff I think it might.
Hope this helps.

I would not recommend an addition of MSB. Its upsampling only makes sound more annoying.
In technical side the upsampling only brings more distortions and quantisation losses.
X-10 is differential buffered inverting stage that with its rise time delay helps to smooth-out the messy output of digital player. Theoretically it already introduces large losses of a signal but still better solution than external upsampling DAC + extra run of wires.