I have owned a Transporter for a few years. I use it to feed the digital input on my universal player which has a better DAC. I did spend some time listening to the balanced analog output and its DAC output was very competitive with the sound of other products available at the time the Transporter was released.
I recall reading that new units can be found for under $1,500. I would be cautious about comparison involving adaptors. Adaptors have a lot of metal in them and if used infrequently, may have never broken in. Also there can be problems with volume drops and hum that are situation specific. The volume control on the Transporter will degrade the sound unless set close to maximum.
If you are interested in future compatability, the guy who designed the Transporter is long gone and Logitech seems to moving towards an Apple inspired product line. The Transporter hardware/software will reprocess 192/24 files as 96/24. Many of the the customer base cannot hear a difference between 96/24 and the higher resolutions. I can - particularily with rips of vinyl. There seems to be little interest in a higher resolution product.
Then, there is the matter of their software. IMO, they don't fix bugs quickly enough or thoroughly regression test their new releases. You have to upgrade to the current release to get bug fixes and may get new bugs and resurrected old bugs along for the ride. If you own other Slim products, the upgrade can effect them all. Also looks like they are downsizing. In a few years you may want a better DAC that the Transporter or sister product may not be able to send a matching high res signal to. All IMO.
Bob