God, this is so subjective, The Siemens silver plate 12au7 is definitely one of my top 3 favorites. Would I recommend this with SS amps? probably not. I say that being a tube guy. This is a very neutral/natural tube. It has extension up the yin yang.It could be to neutral for some systems.
If you are using SS amps and have revealing tone coming thru right now and are looking for a warmer tone to soften the relentless detail of SS, you really have to be the judge here, if it is just a tad to analytical ,then on one extreme the old RCA black plate D getter version from the 1950's era might be your ticket to the tone you seek.This tube does wood bodied instruments ,female vocals, skins of drums as well as any I've heard.Keep in mind though it tends to be the one of those that will have you just staying up late at night listening saying "that sounds wonderful" but it is a full bodied warmer presentation of the spectrum.
Back to the Siemens Silver plate,natural and a complete revelation into what is in the recording, plus it has a wonderful taut bottom end with an extension on the highs that is so superior to any others that air, tinkling of cymbals,and just the clean presentation may make you wonder why any other tube .
Evaluate your systems tone, what would you change? The tone you seek may be such a simple change in tube.
My expertise is not in op-amp rolling, I'll leave that to those in the know.
Side note: we have stopped selling the original DAC. A Mk2 version will be available in about a month. Here are the features.I just didn't want folks PO'd at me that I sell them a DAC and a month later an updated version is available.The cost will be $1050-1100 depending upon my shipping cost. Please keep in mind that just the price of metals are up about 30% across the board . We also have eliminated the volume control on this MK2 version.
1. Sabre ES9018 DAC chip
2. A Sync USB 32/192
3. Dual power transformers separate digital and analog
4. improved tube output stage
5. improved PSU
6. improved signal input section with high frequency relays/resistors module
7. Tube/SS outputs selectable
8. Socketed output op amps
9. improved S/N level
I know that Alex Yeung has had the prototype floating around between his friends Vincent/Vintage Sounds in HK plus those who regularly hang out there and his best friend/man at his wedding Tim/President who runs the HK Audio Tube Club and both are taken back by the significant upgrade in the sound quality from the original.
I follow a lot of the threads here but because of being the distributor of EE I have to very careful of not crossing a line. I hope this isn't.
May Love Always Abide,