The digital output delivers a better, clearer signal, but it does not receive any boost whatsoever from a commercial cd burner.
You have to rely on a properly recorded source in your BR-8... using your individual faders and master-out, in order to get to the desired 0-db level on the burner.
If the music you recorded on your BR-8 peaks out too low (say, around -3 db) then chances are you won't be able to get it up to standard Redbook level.
There are 2 ways to get around that, though.... if you have these options available to you.
CD burners like the Tascam CDRW-700 and the CDRW-5000 have a built-in "db-boost" that commercial burners don't have. That would do the trick for you.
(OR)
If the BR-8 has a "global" type setting that would allow you to put some gain in the output signal... then that would also work. I'm pretty sure the BR-8 doesn't have that option, though.
If neither of these suggestions would work for you, then you can either go analog-out (which doesn't sound quite as good.... but still pretty close).. or, you can record the song optically into the cd burner, feed it back out digitally to the BR-8 (putting it on to the stereo channels at a hotter level) and then feed it back out optically to the cd burner at the "hotter" level.
At that point, you'd also be able to tweak the song a bit (if the BR-8 has a built-in parametric eq and/or a compressor).... basically, in effect, "mastering" the final mix.