The digital mods are mostly centered around improving the internal power supplies that are used for the network chips, CPU, and the digital buffer.
The RCA output connector is replaced with a WBT NextGen and the signal is rerouted for improved transfer.
The power supply work WILL help out the toslink section. The Bybee Purifier is installed in the coax signal path. It will not do anything for toslink.
FWIW, I do prefer the sound of coax over that of glass toslink. For non-critical applications, like the digital audio output of my cable box, toslink is fine. For my SB I use the coax output in my HT system.
While other sources may benefit from Bybee Purifiers downstream of the SB, the SB does benefit from them built into it.
There is no reason you can't have multiple Purifiers in your signal path. I have them on the output of my SB and my DVD player. I have them on the inputs of my speakers. My Equi-Tech balanced power conditioned has then built into the secondary of the transformer. My Ultimate PS has four of them in it. I even built them into my component video cables from the HDTV cable box to my RPTV

I have never heard that particular DAC. I have compared the analog out of a modded SB2 with Sonicap Platinums with no Bybee Purifiers to a Lavry DA-10, both direct into my power amp. I did prefer the sound of the modded SB. TO me, in my system, the Lavry sounded too "PRO AUDIO". It reminded me of the sound of JBL 4311 monitors. Very bright and in your face.
Other folks may prefer this type of sound. The only way you can know for sure is to listen to it in your own system in your own room.
I am afraid I do not have any demos available for audition at this time.