What's wrong with my Cal Delta/Sigma?

Replace the 12AX7 output tube with the low noise version from Cal Labs (~$40). Also, ditch the optical digital interconnect and get a high quality (preferably silver) unshielded .5M digital coaxial interconnect. Set the units aside of one another and not stacked. Do not use vibrapods. Make sure that you NEVER turn off the DAC other than to move / change connections / service the tube, etc.

Had this Combo for 8 years and have experimented much. This is what worked for connection to ARC LS-25 / VT-100.

Another good upgrade is to change the 3 connectors to the upgrade versions from CAL ($60). Don't attempt this on your own unless you are confident in soldering skills, electronic technician abilities, and are very careful. The unit must be totally disassembled, connectors desoldered from PC Board, new connectors mounted in the Bulkhead, and wires run from the new connection to the PC Card. Just another tip...

Hissing -- Most likely the tube needs replaced.

I used AudioQuest power cords with the Ferrite removed...

Tesla is a 'good' tube.... There may better IMHO... It all depends on your system, ears, wallet and the desire to tweek. I liked the stock LN tube from CAL $40 (per my other post), and Kevin Deal has a tube suggestion (which I can't recall) that was also nice for ~$30. I had a NOS Telefunken that was also beautiful, but costly ($125).

Good luck with the tweeks -- I am glad you got your original problem(s) solved. I guess I qualify as a "tuberoller" (my current system is all tubes with the exception of the transport, cables and speakers).