A few pointers, to help you ensure pitch perfect sound out of the Mac.
In your DVD Player, Preferences>Disk Setup>CHECK Disable Dolby Dynamic Range Compression.
Always make certain that all volume settings are at maximum. With the Toslink connected it should default to that, but just check to make certain that you don't have a lower volume setting in SysPref>Sound; Audio MIDI Setup, etc.
Also, make certain that your iTunes isn't using Equalizer - uncheck that.
Likewise, in your iTunes Preferences>Playback - uncheck Sound Enhancer and Sound Check.
Last, but not least, you have to set the proper sample rate for the material you are playing. Usually this is 44.1kHz for CDs, but with certain movie sound formats at higher sample rates you want to set the proper rate to ensure that your Mac isn't reconverting before sending it out.
Set the bit rate at the maximum your setup will permit - usually this is 24bit, though some sound cards permit 32bit. This is just to ensure your various material has enough headroom and doesn't get "downconverted." BeoSystem 3 can handle up to 32bit/192kHz.
Some sound cards set the rate automatically, but usually it has to be done manually. Benchmark has a nice wiki concerning this on Macs.
Now you're set! And you may actually find that your Toshiba got a challenge!
The biggest sinner here is the Dolby Dynamic Range Compression - it's used to help the small speakers in laptops and earbuds not jam up when there are sudden leaps in dynamics; but you don't want that limiter on through your great speakers!
It's also worth it to ensure you are using the right speaker configuration setup for multichannel in Audio MIDI Setup.