I prefer the coax with a good cable. I am using a Virtual Dynamics Master digital cable and it give the most musical sound.
The Virtual Dynamics seems to bring out some of the background instruments into their own space a lot more than the optical glass does. And not only does the does bring it out more, it also makes that instrument livelier and very easy for me to shift my focus to in order to enjoy that musician's emotion or personality.
I am not familiar with the Audio Alchemy device, but I can tell you that if it truly eliminates or reduces jitter, it will help. I am just learning about jitter, but it you direct your jitter reduction at the source, it will reduce the amount of jitter being amplified.
I also have a Mac Mini with a RSA Poiema USB cable that I run into the Valab. I just received it the other day, so I will be comparing that to the other inputs to see where the differences lie.
What type of amp are you using? I am using tubes. I would be interested in hearing how you are enjoying your Teac with your Valab.