[HairyMerkin]

I built a cmoy, and I'm pretty happy with it. However the plan was always to use the cmoy as a precusor to building an o2, but now I'm wondering if I'd benefit from buying a dac that takes optical in, instead?
 
I use a PC (Audigy 2zs)> cmoy > Ms-1i. 

 

[mikeaj]

Depends on the headphones and the CMoy.  Depending on the particular design and board layout and headphones, it may be doing a decent job already.  What headphones are you using?
 
Audigy 2 ZS is kind of ancient, but in general I'd say that a dedicated sound card should still be able to do D/A pretty decently so a better headphone amp is probably a bigger upgrade, for many headphones.  However, IIRC Audigy 2 ZS is from the dark ages of Creative where they hardware resample any content that's 44.1 kHz (pretty much all music ever released), and they don't resample very well.  It may just be worth replacing because of that.  Well to be honest I'm not sure I could say Creative is currently going through the Renaissance, but I don't think they have the sampling rate issue anymore, AFAIK.  

 
Optical is not particularly worth it.  What you should be focusing on is the quality of the DAC and not really the interface.  If you wanted a new external DAC, USB or S/PDIF is not really a big deal.  There are potential small issues with both.  Sometimes you can get slightly better performance using USB, as seen here at the bottom, for example:
http://www.stereophile.com/content/centrance-dacport-usb-headphone-amplifier-measurements
 
If you were to get an amp or build one, it needn't be an O2.  But if the budget is large enough for that, there are lots of worse choices out there.

 

[HairyMerkin]

I'm going to build a gamma1, maybe add the second part to it if I feel like it. If that doesn't sound great then I'll build a better amp.
 
I'm using alessandro music series 1

 

[mikeaj]

Sorry, I don't know how I was so blind I missed the MS-1.  By the way, with the options available, there seems to be less motivation from a performance perspective to DIY a DAC, compared with an amp.  Regardless, it could be interesting as a project in of itself, if you consider the journey to be valuable and not just the end result.