Have the Crack. Have the speedball kit also, but haven't installed it yet. The Crack has a lot going for it. No noise, no hum, no hiss. With good tubes and no microphonics or pings, pops, etc. RCA cleartop 12AU7 and Bendix 6080wb have been delivering an excellent silent background for auditory contrast. Tube rolling opportunities with the Crack also allow tailoring the sound signature. What the ZDSE offers beyond the Crack is a better sense of air around the instruments. The dimensionality and sense of space is much greater. Even though I really enjoy the ZDSE, I won't be selling the Crack. It offers a more rose colored glass presentation of the music. When a recording has certain flaws in the mix or mastering, the Crack has a way of smoothing over those fine details. ZDSE isn't as kind. That is one of the big trade-offs. For well recorded music the ZDSE offers a spectacular presentation.
I also have the O2. Owning both an O2 and a Crack is a great way to cover both accurate and euphonic reproduction without breaking the bank.
Good point, I'm rather limited honestly on what kinds of tube amps I can own. As I have a balanced dac... kinda feel the need to feed that into a balanced Tube which the cheapest would be the LD MK VI+ at about $700 dollars... not to mention the 2 pre amp tubes and $ power tubes that need rolling >.>