Yes there are benefits the UPA-200 / XPA-200 has over the a-100. First it's fully discrete and dual differential, where the a-100 is not. One of the main reasons to go with the a-100 is it already has a volume control. With the UPA-200 you would need a pre amp of some kind. Othere than that as you go up in the Emo line the better the amps get.
Brunk or SWA can build you a pre amp if you DAC doesn't have one.
This is a great place to "start". This will allow you to put more money in a better DAC. The D2 is also a good start. However, if you can spend a bit more maybe look into the MHDT line of DACs - If you want to keep tubes in the chain. http://dhost.info/mhdtlab/mhdt%20dac%20families.htm This will be a step up from the D2
BTG audio is also a great place to get your cable made. I would ask about hardwiring. I'm big on that.
Makes sense. Unfortunately I can't swing anything super fancy yet but I can decide between the UPA-200 and a preamp and a TBI Millenia. Which would you choose between those?