My previous comments were meant to offer some idea of the differences between the MacMini and the MacBook Pro when using the internal DAC and audio outputs.
And I would agree, when you use external DACs it is possible to get really top notch sound, mostly depending on the DAC you connect to the digital output.
One IMPORTANT point is that the MacBook PRO, like the MacMini, has the firewire 800 output and the regular MacBook does NOT have firewire. This may be important as new firewire DACs show up on the market (e.g. the Weiss unit).
Also, to further bolster why maybe get the MacBook Pro: one of the possibilities the MacBook Pro offers over the MacMini is full battery powered operation. If you combine it with something like the RedWine audio or Virtue Audio gear (both are spectacularly good), all battery powered, you could very well end up with state of the art sound.
The power supply issue IS an important one, and the battery operated gear sounds so very good, it makes me wonder if there is a way of putting together a good battery supply for the MacMini too... The improvements I have heard on the Virtue when running on batteries compared to even the "best" (super high quality) switching or linear power supplies are so huge that it makes me think this is worth exploring.
SO, especially if you hope to run on batteries AND use an external USB or Firewire DAC, then yes the MBP makes a compelling choice, albeit at a 600USD premium.