It did seem like the 6AS7G position didn't make much difference - 2 6AS7G and 4 6H13C, until I tried a 7236 variant, then there was a big difference
There are a few Sylvania 7236 pairs on eBay right now for $40/tube in pairs - I've successfull negotiated a single tube purchase 4 out of 5 times, it might be worth asking the seller if they have a single 7236 available for sale.
Warning Update: It looks like the Sylvania 7236's aren't actually a "real" 7236, some think they are Sylvania 6080's relabeled as 7236's as they don't sound any different than Sylvania 6080's. I'm going to stick with the Tung-Sol/Cetron 7236's myself.
I paid $59 for my single NOS Cetron 7236:
NOS VINTAGE TUNG SOL CETRON 7236 5998 TUBE WOO HEADAMPS PAIRS AVAILABLE
And, over time I've seen that is about the right price - $40-$50/tube in a pair - but also some go for much more - there are a Tung-Sol 7236 and Sylvania 7236 auctions going with $108 as a "buy it now" from the same seller:
https://www.ebay.com/itm/284673358577?hash=item4247dabaf1:g:VisAAOSwuThfyY0e
https://www.ebay.com/itm/284673358504?hash=item4247dabaa8:g:SH0AAOSwPjxfxvRL
I waited until there was a NIB 7236 tube for the right price, and I am very happy I did get mine, it is an amazing sounding upgrade from the 6AS7G - I'll be hunting for another. BTW, I've heard that all of the 7236 tubes were made by Tung-Sol who was later bought by Cetron:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=7236+tube&_sacat=0
The list of 6AS7G variants in this post was very helpful, with the 7236, 5998, 421A the top compatible variants that reports say make a big difference:
https://www.head-fi.org/threads/for-6as7g-tube-rollers-here.410326/post-5441008
The only 2 steps up would require verifying by Xduoo that the TA-26 could handle the doubled current draw:
6528 - equiv to 5998 but very high gain at 9, twice the current load. Make sure you amp can handle it
6336 - like 6528 but low gain at 2, still twice current load. Make sure you amp can handle it
I've asked Xduoo directly about that question, if those particular tubes can be safely used in the TA-26, but Xduoo haven't tried them and couldn't answer to their specific support in the TA-26. I've thought about getting a couple and sending them to Xduoo with the warnings about what to expect - specification sheets and links to others reports of modifications needed in other amplifiers. But, for now I'm happy with what I've got, and will suggest someone else might want to take up the baton and try this with Xduoo themselves.
Recently I've found reports of use of an even rarer variant, not on the list, and it does sound compatible and promising:
Tung Sol 7802 -
https://frank.pocnet.net/sheets/077/7/7802.pdf
The 7208 has been reported to work in amps similar to the TA-26 without modification, and by others with a small mod to avoid self-oscillation due to 20,000 mho of 7208 vs "standard" values of 1/3 (6AS7G)- 1/2 (5998/421A/7236);
https://forum.bottlehead.com/index.php?topic=3664.0
One sold back in December on eBay:
https://www.ebay.com/sch/i.html?_from=R40&_nkw=Tung+Sol+7802&_sacat=0&rt=nc&LH_Sold=1&LH_Complete=1
https://www.ebay.com/itm/324959603962?hash=item4ba91a0cfa:g:YAAAAOSw8mJhxoGv&nma=true&si=RlLfpB3BERKYRO376Ku9L2kwQts%3D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557
That eBay 7208 product description reminds me that I am also looking for a
NIB Bendix 6080WB (
also here) with Large Graphite Plates and thick Ceramic Spacers as described here:
"Extremely scarce Tung-Sol 7802WB tube, labeled for Lewis & Kaufman, with original box. This is the only one of these that I've had in decades of tube collecting. Often used in place of the 6AS7 and 6080 in headphone amps. This is the "WB" version, which like its 6080WB counterpart, is made with warp-proof graphite plates, ceramic spacers, heavy glass, and other features that result in a very durable tube. Excellent balance between sections"
Check out the innards of that 7208, the crazy coating on the plates, the triple thick ceramic spacers instead of Mica, the thick copper rods, and the thick glass... one awesome looking tube:
Here is a Bendix 6080WB which has similar construction:
https://www.ebay.co.uk/itm/Bendix-Matched-Pair-CEA-6080WB-Tubes-Carbon-Plate-3-Mica-039-s-Excellent-NOS-6080-wb-/304193734092?hash=item46d35c05cc:g:yk4AAOSwJSthcRm9&nma=true&si=vwB3EKVpYaB6f10cNOM9uOs9pWs%3D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557
I think I'll buy another 7236, maybe a Tung-Sol labeled to see if they sound different than the Cetron 7236...it really does change the sound of the TA-26, similar as described in that 6AS7G variant list:
7236 - medium gain - great bass, fast and dynamic. Almost SS sounding
The Cetron (Tung-Sol) 7236 adds back a little of the SS sharpness, focus, and adds powerful impact, while giving that wide stage eufonic sound I love.
Warning Update: It looks like the Sylvania 7236's aren't actually a "real" 7236, some think they are Sylvania 6080's relabeled as 7236's as they don't sound any different than Sylvania 6080's. I'm going to stick with the Tung-Sol/Cetron 7236's myself.