@ghdprentice said,
"In high end systems they don’t sound like tubes…they are used to enable incredibly natural musical high fidelity sound".
If there ever was a definitive statement, succinctly put, this summarizes the 'tube sound question' in one sentence. Bravo!
I boil it down to one word - realism - when someone asks. :-)
To the OP:
I have an all tube system, the DAC is a vintage solid state Theta, but my preamp has an interface board built in that is dedicated to its output into the pre. It is the RFC circuit* designed by Dennis Had of Cary Audio. In fact, my preamp is Cary as are my monoblocks. Sublime realism coming through the Maggies.
If I were to upgrade my DAC, the Jolida/Black Ice would be a top contender - a great sounding DAC 'specially with a tube chain and cost effective to boot. Frankly, with the Theta and the RFC circuit, the realism is astonishing. Mike Moffit was on the cutting edge then (and still is) of digital to analog conversion.
The take-away: The right tubes when done right (good transformers are a must) = Realism (to me, anyway). No 'tubey' sound. If so, something isn't quite right.
*For those interested: The RFC circuit was a Reverse Phase Cancelling circuit that stripped out the ultrasonic noise (above the Nyquist limit), which is produced by DAC chips, so the analog section does not amplify it. It was accomplished using two 12ax7 tubes. The circuit literally reversed the phase of this inaudible noise and cancelled it out. It wreaked havoc on the analog sections and made digital sound 'edgy'. In the salad days of digital (to use Dick Olsher's phrase), this was ground-breaking. The Theta was used as the test bed for this circuit which I also bought. Going forward, this was accomplished in the digital domain and really helped tame digital. In point of fact, that patent proved very handsome...so now you know.
On a personal note, Dennis is a very talented tube engineer. I discovered in my first conversation he was a fellow Ham and the designer and builder of the Dentron line of tube R/F amps which he sold to start Cary, which is when I interacted with him. He custom-tweaked my Cary chain starting with the installation of his RFC circuit, which was the basis for a dedicated line preamp, and I have never looked back.
The only other addition to the chain has been the Jolida Foz SS-X, a tube-based line level device, which I wrote about in a separate thread. I have my system in a very small room and the SS-X addressed the issue. So, adding tubes does not degrade the sound if done right~!
Thanks...