Thanks for your extensive reply. As always, it seems!
...
I find if I answer in short without supporting info, worse conclusions can be jumped to due to inadequate information - and their inability to read my mind.
So I prefer to go into detail in my responses vs trying to repair a misunderstanding in many following posts. So far that works well, unless the person isn't interested or doesn't read well. And then in the long term my longer responses are helpful to others that come across them later.
I'm answering to help, and if I don't cover things well at first edit, I often come back and edit my post(s) to fill in words, details, and add links to firm up the specifics. And, I come back sometimes with "Updates".
Have you been running the amp from a balanced or unbalanced input? I really think — hope — that it’s that. Also because rolling the stock tubes made no discernible difference, I think it’s elsewhere in the chain.
I've run the TA-20 in both XLR and RCA, and I prefer XLR on the TA-20 but on the Topping A90 I prefer RCA...it isn't a big difference for either, it is a preference based on longer term listening, and I had to pick one or the other in order to drive both at the same time from my Topping D90 MQA DAC.
Right now I am driving the TA-20 via XLR from the Topping D90 XLR output, and the TA-26 via the RCA output of the Topping D90 - the D90 can be set to output to either RCA or XLR or both at the same time.
Again the difference between the XLR and RCA input of the TA-20 for me is much less than between good and great tubes, and between a broken in TA-20/tubes - and the sharp edged harshness of new unused tubes -
harshness which can be extreme with some new tubes!
I am fully aware that it takes a good number of hours of burn-in for tubes and caps to come to life. I did however have much higher expectations even on initial listening, based off of many reviews with positive first findings. On forums like this one as well as Zeos’ super positive review. My initial impressions are far from that.
Well, not so much to come to life as to remove the rough edges of harshness from the new tubes/equipment. There are other dimensions to the changes,
but you are reporting - in my estimation - the harshness of distorted edges from the new tubes and/or new TA-20, both not yet broken in.
I've experienced it many times with new tubes on the TA-20 and now on the TA-26. The tubes do at first have good qualities and bad qualities. Some/most tubes shed the bad sharp/harshness qualities over time and as I and many call it - mellow out. And, the sound stage can widen and the overall frequency balance can improve. Each tube is different in the changes over burn-in, but so far none have become worse sounding.
I also experience this burn-in harshness with IEMs/headphones - as with the Sendyaudio Aiva Zeos uses in the TA-20 review where he says you really need a Hybrid amp for Planars - which I have found isn't always the case - the TA-26 drives my Hifiman HE6SE V2 quite well - even though it is very inefficient and has low impedance.
I've A/B'd both the Hifiman HE6SE V2 and Hifiman Ananda between the TA-20 and TA-26, and with certain tubes in the TA-26 - specifically the 1976 Cetron 7236 power tube and the 1944 Kenrad VT-231 Black Beauty (Black Glass) the TA-26 sounds just as great as the TA-20 with Haltron E80CC (Heerlen made) or E180CC Heerlen made tubes.
As for the Sylvania’s, I will not argue that the Heerlen tubes are a another step up surely. Still, there are *many* who think the Sylvania’s make the top of the list just beneath that, having posted reviews counter to the one you quoted. Personal preference of course but I’m holding on to the thought that these are some good tubes which should not produce such unpleasant distortion by a far stretch.
In my experience my first tubes - as carefully researched and selected as I could make them - didn't hold up over time, and like the Sylvania 6189W your early tubes may similarly be near the bottom of your list, after you run through a lot more tubes, hunting for better and better tubes.
tubemazestore - a longtime tube seller on eBay that also has a
tube review site with 5000+ reviews submitted as of August 7, 2020 - see top rated 12AU7 list below.
The Sylvania 6189W Triple Mica received 3.84 out of 5 - (32 votes, average: 3.84 out of 5) Is your 6189W a 3 Mica build?, one on the bottom of the stack and 2 with a short gap on top? Most 12AU7's (and other 12xx7's variants) are built with 2 Mica, one top and one bottom, but the best sounding are typically the 3 Mica variant also usually built with stronger posts and tie points to keep the tube rigid in the glass envelope - those more solid builds reduce the noise and microphonics.
https://tubemaze.info/sylvania-618912au7wa-gray-plates-triple-mica/
Here are the tubemaze.info site's top rated 12AU7 tubes as of August 7th, 2020:
https://tubemaze.info/the-best-12au7-tubes/
"Which 12au7/5814 tube won the right to be called the best and the worst?
Updated August 7th, 2020
Based on user ratings in the last 8 years and over 5000 ratings submitted, here is the list of winners and losers. The system is based on 5 stars. We will treat 5814, 7316 and 12au7 in the same category – since they are the same tubes, just different quality.
Old Stock 12au7
Winners
Amperex 7316 Long Plates Foil Getter (4.8)
Mullard 12au7 Long Plates []-getter (4.6)
Mazda 12au7 Gray Plates (4.57)
Mullard 12au7 10M Series (4.5)
Tung-sol 12au7 Black Glass (4.43)
No surprises here, all of them are crazy good tubes. Amperex 7316 with long plates still reigns supreme.
The 3-Mica Sylvania 6189 with 3.84 rating didn't make the top 5, and probably not the top 10...note below the PSVANE 12AU7-T Mk.1 received an average of 3.18!, the site search doesn't find the PSVANE 12AU7-T Mk II - too new?, but it did find the PSVANE 12AX7-T MK II which received a 3.38. So perhaps I would enjoy the Sylvania 3.85 more than the PSVANE 12AU7-T Mk II, but in any case I have a far more enjoyable 12AU7 in the Tung-Sol 12AU7 Black Glass 4.43 rating - which are my current best sounding 12AU7 tubes - I recommend to always get backup's of your favorite tubes - if possible
Losers
Sylvania 12au7 Short Black Plates (3.0)
Sylvania 12au7 Gray Plates (3.0)
Mullard M8136/12au7 Box plates England (3.28)
Now that is a surprise, based on 39 reviews submitted, Mullard M8136 took the third place from the bottom! The very unexpected outcome for this tube. Interestingly enough, Mullard CV4003/12au7 Box Plates earned a very respectable rating of 4.4
New Production 12au7
Winners
Well, there are really no winners in this category for new production tubes, all of them rated very poorly. - That is my experience as well, while "nice" they really don't do the TA-20 justice.
Losers
ShuGuang/Ruby 12au7 Gray Plates (2.0) - AFAIK these are the stock tubes provided with the TA-20.
Psvane 12au7-T (3.18) - These are the predecessors to my PSVANE 12AU7-T Mk II's and PSVANE 12AU7-S Mk I's.
Electro-Harmonix 12au7 (3.22)
And again ShuGuang 12au7 takes the lead on being the worst of the worst – 2.0 is quite an achievement – again, too bad that a lot of manufacturers choose to ship their equipment with this tube. - IDK why Xduoo keeps using these as the stock tubes when the much better Chinese PSVANE tubes are available now...some Chinese amp makers offer a PSVANE tube upgrade option - perhaps Xduoo could add that as an option for the TA-20 / TA-26 / ?? models?
DISCLAIMER: This is not a definitive list of all best and worst 12au7. It’s purely based on over 5,000 ratings submitted by the users of this site."
My comments added to the article quoted above are in Italic Bold....
Again, Zeos loved the TA-20 on first listen even with stock tubes.
I’ll continue listening on this unit and wait for my Ares to arrive, and postpone final judgement until I’ve hooked that up balanced.
Zeos was comparing the difference between the TA-10 single-ended sound vs the TA-20 Balanced sound, and as far as the stock tubes Zeos says "I think if you swapped tubes you could have an "end game" "- and Zeos's comment wasn't that stock tubes sounded better, but that Balanced sounded better - more specifically the TA-20 sounded better with stock tubes than the TA-10 with burned-in swapped tubes.
But I don't think Zeos did a good job qualifying that comparison if you got out of it that the stock tubes were great sounding, or that they were responsible for what Zeos was praising - it wasn't the sound of the stock tubes it was the difference in sound presentation between the "single-ended" TA-10 and the "balanced" design of the TA-20.
As above the TA-20 stock tubes got a 2.0 rating and I agree that they are very harsh sounding on their own, and not at all enjoyable, that is why I didn't listen more than a short while (minutes) to them before rolling in tubes that sounded much better than the stock TA-20 tubes.
Zeos's comments as above start here:
Xduoo TA-20 _(Z Reviews)_ Truly Balanced
The TA-20 Balanced sound sounds much different than the "Pulled to the Center" single-ended TA-26 sound...at least initially. Now that I've listened to the TA-26 for a few months that "sucked to the center sound" I had with the stock tubes and unburned in TA-26 is completely gone.
If you continue rolling tubes, and continue using the TA-20 all day long - keep it playing music even if you aren't listening to it to get it through full burn-in, you will find a much more musical and melodious sound from your TA-20 too
Even though it's a hybrid and not a full tube amp, I'd give it around 100 hours before making a final decision. My TA-30 sounded very closed in at first, also.
Totally agree, and for me the TA-20 sound kept improving all during the first couple of months of use, and that was about the time I found the Tung-Sol 12AU7 Black Glass tubes and stopped listening to the Topping A90 completely - except for specific comparisons to answer questions posed to me online. Now, the A90 is sitting unplugged under the TA-20...
I would expect the TA-30 to need as much burn-in time too.
For me the stock TA-26 was stuck on stock tubes for a couple of weeks until I could get new tubes plugged in, and I often went back to my TA-20 simply because I thought the TA-26 stock tubes as still new sounded too harsh - but, with the TA-26 wide sound stage and "airy tubiness" being much better than most TA-20 tubes sound, even while the harshness played out during burn-in.
@roderickvd -
I also should mention that for both the TA-20 and TA-26 with new tubes I find it helps to reduce the XLR 4.0V drive output from the D90 DAC as it reduces the "edgy-ness" or harshness at listening volume with new tubes. With new tubes not yet burned in, on the Topping D90 I reduce the XLR 4.0V output by setting a -15dB or -20dB attenuation - with the PC source audio still set at 100% output to maintain analog Signal-to-Noise audio integrity, and that helps to reduce the harshness during burn-in. After the amp/tubes burn in I run the D90 DAC at 0.0dB output attenuation - like right now.
The DAC output attenuation, even as little as -3.0dB, -6.0dB or -9.0dB might be enough to help you reduce the harshness you are hearing during new TA-20 / new tubes burn-in. Trying playing around with the DAC attenuation starting at -3.0dB and increase attenuation until the harshness is reduced as well as possible. Of course, increase the volume on the TA-20 as needed to return the listening level to where you like it.
Nothing beats actual hours burn-in time, but I can make the harshness more tolerable during burn-in by reducing the DAC XLR 4.0V drive input to the amp.
Additional detail for reducing DAC output: This probably only applies to XLR output @4.0V or more as with the Topping D90/D90SE. If you are feeding the TA-20 / TA-26 via RCA 2.0V or less it won't make a difference.
Thanks to @roderickvd for his feedback, I've updated the info above to reflect his experience
Update: tubemazestore was paused for a while, but it is back online again, not very much is currently listed, I hope he brings back more of his inventory online soon 
although a TA-26 indeed is on my list for later.
