Ideally, I'd like to connect my hard drive via USB to a preamp/DAC and then to an amp that would power my bookshelf speakers. I don't think there is an all in one that exists.
No - the already mentioned Squeezebox might be your best option. But read on....
I'm not opposed to using a laptop in place of the pre/DAC, but if there is something that would do what I want without using a laptop, that would be ideal.
Again, the Squeezebox or a TEAC WAP-****. Also, the Cambridge NP30 and Olive range both allow USB-host, but might be above budget.
The next biggest question is if something like the Squeezebox touch or some other pre/DAC exists that would be able to directly read from a 2TB USB drive.
EDIT: To clarify, the Squeezebox Touch does support drives over USB, but don't mention size limitations. I haven't found anything like the Touch that will read directly from a USB drive without the computer in the mix.
Now this is where it gets wonky. The Squeezebox is really meant for wireless access - hooking up a USB drive directly is not the way to go. The problem is that for large(r) collections (> 300 GB), the Squeezebox takes a really loooong time to scan/catalog everything on the drive. Of course if
1) you leave your collection as-is (i.e., no file addition or deletion) for a long time and
2) you leave your setup powered on
this might not be a problem.
Otherwise, I think your best bet is to get a NAS or (since you already bought the drives) a Iomega iConnect Wireless.
As for pre-amp/DAC, it depends what your budget is - the Squeezebox's internal DAC is pretty decent, but a NuForce UDAC-2 is only $130 or so. FiiO's are pretty good value for money as well.