Toslink

The history behind that is that TOSLink transmitters would often have measurably high jitter. But if your streamer does not exhibit that or if your DAC's receiver mitigates jitter then that shouldn't be an issue for you. The other benefit of TOSLink versus S/PDIF or USB is that there is no electrical connection between the transmitter and receiver.

Sometimes Toslink may not handle sample rates higher than 96 kHz. If the connection is iffy at a sample rate of 176.4 or 192 kHz, usually what happens is that the output will sporadically mute because synchronization is being lost between the streamer and the DAC.

SPDIF usually handles up to 192 kHz PCM and DSD64 if using the DSD-over-PCM (DoP) protocol while USB can handle up to 384 kHz PCM / DSD128 (DoP) or even 768 kHz PCM / DSD256 (DoP). Some DACs offer native DSD playback over the USB interface, though the streamer must support the feature with that particular DAC.

So it really boils down to what type of sampling rates you want supported and whether high bitrate DSD is important to you.

I think Toslink is still most likely to be an inferior interconnect. It has two extra stages in conversion that are unnecessary. Also, for most companies it is an add on just in case… so stock electrical -> optical and back. It is not so much the transmission media but unnecessary conversions.

SP/DIF has been the oldest and used for decades… then USB… needing completely different receiving processing and optical. AES is a newer, made for high end audio which I thing is just balanced SP/DIF connection.

As far as connections, in general, AES… preferred, SP/DIF , then USB (depending on DAC) then optical.

If the DAC is optical only it is likely an inferior DAC.

I think it very much depends on the DAC. If this is an older DAC, then probably. Older DACs, before USB asynchronous was ubiquitous did not have the jitter and timing stability that they do in the last 10 years or so.

If you are getting an older DAC you’ll find that upsampling really will help the performance.

Kind of for this reason I normally try to steer users away from them. The Wyred4Sound Remedy is a very good solution if your streamer doesn't already do upsampling internally.