Thanks Martin
The Standard build comes with analogue outs to your amp, and is fitted with a Spdif coax out capable of 24/192 to your Dac.
If someone wants to use their own dac, you can use the inbuilt spdif output or standard USB sockets.
You can specify the upgraded USB output in a Server build but this is an optional extra.
If someone wants the upgraded USB and has no use for the analogue outs they can save a bit of money on the upgrade by letting us know, and I won't fit the facility for analogue outs.
If your using USB I'd absolutely say the upgrade is worth it.
The improvement between a standard USB socket and the upgraded one (even in to a good asynchronous Dac) is very noticeable.
A big part of my thinking when I've been testing different configs for the server, was how the things that ACTUALLY AFFECT SOUND QUALITY can be isolated from any potential mush being generated inside the server case.
Isolating the USB from the rest of the system made a noticable improvement.
It's the way to get best possible sound quality from the box if your using a USB Dac.
That answer yours too Ali?
You can use standard USB to connect to any Dac with USB input, but it won't be as good as the upgraded USB option.