Musical Fidelity X-10D

At present I'm running a pair of Pioneer DV-727Fs into an SAE 2100L pre-amp via a Musical Fidelity X-10D. I'm considering the insertion of an MSB DAC III, with the upsampling upgrade, into the mix. Would I still benefit by keeping the X-10D, or will the DAC III make it redundant?
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.
Using an X-10D is kind of like filling a gaping wound with sand i.e. you've stopped the bleeding but you've got a festering sore waiting to explode.

Having said that, i agree with Marakanetz' post. The X-10D "works" by rounding off the leading edge of notes ( reduces harshness and glare ) and increasing the level of "ear pleasing" distortions. Going to the MSB is technically a big step forward, but it is not the most pleasant sounding DAC and can tend to sound a bit hard and glaring when used in conjunction with most poorly recorded cd's. "Revealing" or not, having a system that is unpleasant to listen to is not very fun or enjoyable.

Obviously, the aforementioned problems are what you are currently trying to avoid by using the X-10D. While installing the X-10D would combat those characteristics in the MSB, you're back to using a band-aid rather than curing the problem and really advancing your system. As such, i would seek out other methods to improve your sound rather than investing in the MSB. Just bare in mind that your system DOES function as a system, so changing one component by itself may not give you exactly what you are looking for until you've balanced out the various "weak links" that you may encounter along the way. Sean