CAL Alpha DAC: Stock vs. 24/96 upgrade

The following webpage contains a chart that will tell you what to expect from some popular 12AX7 varients (https://www.tubeworld.com/12ax7.htm). I don't like the Btitish coloration of the Mullard CV4004 personally. My favorite was the Telefunken ribbed plate(NOS) in the Alpha and Sigma. I owned both for a number of years to go with my Delta. I'd also suggest tube rolling and trying a cable like the the Kimber Orchid before the upgrade. If you do get an Orchid: be certain to get one of at least 1m. For some reason the shorter lengths don't get it(personal experience). (http://www.kimber.com/Products/DigitalCables/Orchid/Orchid.aspx)

I don't know what you're using for an interconnect. If it's not something outstanding- this will vastly improve your combo's musicality (http://cls.audiogon.com/cgi-bin/cls.pl?cabldgtl&1206926128) I owned an Icon II, and when I went to the Delta/Sigma- my son inherited the Icon. When I got the Alpha(and installed the Telefunkens), it was another audible magnitude of improvement(toward the sound of live).