Glass Fiber AT&T ST or AES/EBU XLR ?

I can't speak to the Theta but I can tell you that I went to 2 Aural Symphonics Optimism v2 cables between a 270SE and 27. The clock signal cable was a real improvement. The music signal, IMO, has become extremely clean and analytical. I would call it incisive, not harsh just extremely detailed. For my taste, maybe a little too detailed. I am considering a new cable (XLR) with a touch more warmth. The problem is that there are so many digital cables available that it's mind boggling. JMO here but if I were you I would stick to XLR but maybe try a few others rather than pay to upgrade the Theta. You could purchase a great XLR cable for the cost of a Optimism v2 and the Theta upgrade.

Tomer_tsin in my experience with the AT&T cables they are usually sonically inferior to the aes/ebu balanced cable the only exception being when the AT&T is being used to transfer the data and clocklink signals between Wadia 270 transport and 27ix converter. being your converter is the Theta I wouldn't bother with AT&T cables.
Rja I suggest you unhook your clocklink AT&T cable and listen to what losing clocklink sounds like I'm sure you will not like it's effects. The aes/ebu does not have the clocklink advantage and is truly no contest on the Wadia versus the clocklink

Each system is different My Audio Logic DAC sounds sweeter with RCA digital cables then the XLR ones but there is s slight decrease in detail So try them for yourself

Happy Listeing

Mejames,
Are you saying the Wadia will lose clocklink with XLR? I was under the impression that clocklink will work with other connections. Am I wrong here? Of course I will leave the AT&T between the units for the clock signal. Maybe I need to go back and re-read the manuals.

RJA yes clocklink only works by using both AT&T cables. 1 cable has the clock data the other has the CD word data only. Believe the balanced cable has both the clock and CD information combined into it.