I would add that some 500 series racks come with features beyond a power supply and linkage, including a variety of routing schemes. and I theenk one model either comes with a headphone amp or you can add one.
Burl Audio, in particular, makes a dandy AD/DA and two equally awesome preamps for the format. Some come with beefier than standard power supplies, as well. You should definitely look for DB25 connectors with Tascam pinout; it's pretty much a standard, now and will save you big bucks on cabling whether you use a patchbay or not..
Andy and I don't always have the same perspective, but the mission is accomplished if it gives you "a much better understanding of the choices I need to make"! That's what it's all about.
