I plan to use HDTracks and want to optimize audio quality. I have an excellent DAC that takes both USB and Optical Toslink. I have several questions: Does USB or Toslink provide better sound? Do I worry about audio cards or use dragonfly if I go USB? Would a computer with solid state storage outperform a computer with a hard drive? Any other guidance and recommendations are heartily requested. I want to invest to get the best possible audio quality.
Depending on your application, USB and optical SPDIF/Toslink both have advantages. Not all DACs/sources support 192KHz playback over optical SPDIF, and if you plan to listen to multichannel music (5.1 or 7.1) Toslink very seriously limits your choices (toslink can only do multichannel in certain compressed formats). The main advantage of Toslink is that it provides better electrical isolation, which may or may not matter depending on your specific setup and the design of your DAC.
If you know you're only planning to ever listen to stereo music, and you know that your DAC supports high sampling rates over optical, then Toslink may be a good choice. Otherwise, USB is probably a better choice. I personally use USB because I need to be able to reliably output more than 2 channels, and I like the flexibility it gives me.
Another advantage of USB is that you don't need a soundcard or dragonfly (Which is itself a DAC). If your existing DAC accepts usb input, you can just plug it into your computer directly; you don't need anything else in between the PC and the DAC. If you plan to use Toslink, and your computer doesn't have an optical out built in, you'll need to find some kind of sound card that has a Toslink out, which may add to the cost for limited benefit.
In terms of audio quality, I don't think SSD vs. HDD makes a real difference, but in conventional terms of computer performance a PC with an SSD for a boot disk will "outperform" the same system with an HDD for a bootdisk. It makes the system as a whole more responsive, and has no moving parts so there's less noise from the computer. But SSD storage is usually too expensive to use for bulk media storage and doesn't provide much benefit in that application. It's best used as a place to put the OS and commonly used software.