Looking for a nice affordable desktop amp

That's because USB audio tech is still evolving. There are too many people who went to explore asynchronous when it came out, dumping other DACs while the DACs that didn't have it didn't garner as much attention (except maybe the ODAC). With a separate USB module it will be modular and you can just get the new board and swap it in when some new USB audio tech comes out.

As for why it would be $100, it's hard to say, but business models are complicated that it's not always a scam (or as some say, you'd be led to assume it isn't). But the thing is if it costs that much for them to produce it, you have the option of not having it, instead of selling an amp that has it for $100 more and you're using a transport that doesn't need it - like a computer with SPDIF output, a traditional CD transport, or even a music server like Musical Fidelity's M1 Clic.

I would say give the M/M stack a chance if you want to keep your budget around 200.

If you want a larger budget, go with something like Audioengine D1+ Matrix M-Stage. Since you have a D5K I would personally say stay away from tubes. If you're curious though, Bravo and Little Dot have nice models for a decent price. The Bravo you'll have to mod. I've heard the Asgard is a very good choice as well, you would have to research the differences between the two.

The reason USB is an option on those (and costs $100) is (warning: recalling from memory), a lot of people use SPDIF which is in a lot of cases superior, and using async hi res USB requires drivers, and a new clock if I remember. In other words, unless you NEED usb, just stick with the base model. Don't have SPDIF? Get yourself a sound card for cheapsies with opt/coax, or get a USB/SPDIF converter.