The XP8000 has been replaced by the XP8000A. I've not thoroughly researched the XP8000A, but at first glance, it seems to be identical in terms of features and performance.
The XP18000 is a much larger battery pack that has been replaced by the XP18000A.
As to your question - I would say you should not consider spending the money for a higher voltage external power supply unless you have relatively inefficient headphones that can enjoy an increase in power output from the amp.
They're pretty expensive for what they are. I've bought a rechargeable battery and charger so I'll see how that fares before I invest in something else.