DCS puccini+clock or spectral 4000

The Puccini is a sweet sounding unit but personally I would balk at the price. I don't think you need to spend that much for really good digital.

I have not heard the spectral 4000 but I have heard the DCS puccini+clock. I did A/B against the ayre c5xe and I thought it was much better and more analog sounding. I found that red book cd's sounded excellent. Relaxed, mellow with tons of detail. The Ayre was more forward compared to the puccini and to vinyl. I even a/b red book cd against very high quality analog rig and I found this DCS set up to compare favorably.
The puccini also has digital inputs and I plan on running my IPOD to the DAC. Not sure if spectral has that or if it is even something you care about.
I liked it so much that I placed an order to buy the Puccini however I could not afford the clock at this time. I would not hesitate to buy.

I am an inveterate tube roller and some circuits will not respond to tube changes.
I tried to cool off a highly modded ARC SP6B pre amp. It already had the reputatation of the "legendarby gain gone wild". You can't simply swap lower mu tubes to get a more viable midrange and travel on the volume pot. It takes 12AX7s x 6 and only Only 2 are in supposedly real meaningful gain positions.
Try as I might for example with top of the heap 5751s -2AU7s almost passed enough current to burn up my voice coils, the amp seemed to care less. The result was a change in tone very, little if any perceptable change in gain.

Mechans,

I've found the mhdt Paradisea tube DAC, which uses a single tube in the output stage I believe, changes sound very dramatically with different tubes. I've heard this specifically using a NOS Tung Sol tube in place of the stock GE tube. The difference is at least as pronounced in my system as any other component or cable change I have tried.

I have no experience rolling tubes in other devices though. I have no inclination to date to try it with the ARC sp16 tube pre in my system, which uses 6 tubes total and I suspect is quite a bit more complex to tune this way.