How good of a DAC is the Logitech Transporter

The Touch is fine, but it is no match for the Transporter, not really even close. Also, things get even better still, when using the Transporter balanced outputs...

I believe a USB solution would still ultimately involve an S/PDIF conversion operation - and thus there would be no reason to use USB with a Transporter. A main feature of these SlimDevices units is to avoid any such S/PDIF conversions - the network connection avoids S/PDIF and gives an optimum connection medium.

Also, regarding whether there are "better" AKM dac chips available, keep in mind that Sean Adams (designer of the Transporter) implemented the chosen AKM dac chip in a very optimized way. Here's a quote-snip from him:

Sean Adams: "However, in the case of Transporter there is a very large improvement over the manufacturer's specs because we are using the chip in a far more advanced design than AKM's evaluation board. For the THD+N figures this is especially due to the jung regs and the selection of certain passives in the amps/filters. The clocks are very important too, but this isn't meaningfully reflected in THD+N tests."