I own a highly modded ZXR, but I wanted to give a little input on this question since I own 3 different home setups for headphones, lol.
Option 1: If you run an optical cable from the SBZ you won't be getting the benefits of the SBZ's DAC (analog sound); you are sending an encoded (digital) signal to another DAC if you use optical. This is a good thing if your highest quality DAC isn't the SBZ, but if it is, see the 2nd option.
Option 2: If you run RCA cables from the SBZ to the Magni 2 you will be getting the raw output from the SBZ (already been through DAC on the SBZ) with all the features of the sound card. I run this setup sometimes when SPDIF won't work right in some software with my Aune T1. So I just run two RCA cables from my ZXR to my Little Bear P5-1 -> Magni 2 or E12.
Option 3: You can also run from the amplified headphone out to another amplifier of better quality, but I prefer not to double amp headphones when you can avoid it. This is due to risk of damage to the headphones if crap hits the fan (both volumes @ 100% w/ high gain). You will see people recommending this to maintain the headphone effects, but I recommend using the RCA (line out) method mentioned above over this unless there is something really special offered only out of the headphone jack. If so make sure you never turn your SBZ above say 50% volume, use your 2nd amp as the volume control. I still highly discourage doing this as hum and cross talk is a possibility.
Most of this is related to information I know to be true on the ZXR and I believe it applies across the board for Z series, someone correct me if I am wrong.