you wrote: while the Hifiman HE6SE v2 is driven better by the TA-20, but are you referring to BAL or SE?
It doesn't matter, the TA-20 specifications show 2000mw for both Balanced and Single-ended. To me, between the two choices, there is a center focus pull to the Single-ended sound, so I prefer the Balanced mode sound on the TA-20 and have converted all of my cables to 4.4mm or 4-pin balanced because of the higher power delivery on the FiiO M15 and Topping A90, and 2.5mm on the BTR5. The TA-20 has the same power delivery on both 4-pin balanced and 6.35mm Single-Ended.
On the TA-26 there is only one choice, Single-ended, but the image presentation doesn't sound to me as "pulled in to the Center" as Single-ended on the TA-20 - after listening for a couple of dozen hours my ears are used to the presentation, and Single-Ended sounds as wide and detailed as Balanced on the TA-20.
best pairings headphones-TA-20?
The whole reason I bought the TA-20 is due to the Hybrid Tube/Solid-State design, which drives a wider range of IEM/headphone impedance, so there is less of a "compatibility" limitation with the TA-20 (8ohm-600ohm) as I've found on the TA-26 (60ohm-600ohm).
The 16 ohm DT770 didn't "sound right" on the TA-26 but the DT770 16 ohm sounds great on the TA-20. The 600 ohm DT880SE sounds "amazing" on the TA-26, and only "good" on the TA-20, after comparison with the TA-26. The TA-20 doesn't have enough power at 600ohm to make the 600 ohm headphone "pop" like on the TA-26. I'm looking forward to hearing my Sennheiser 120ohm/300ohm headphones on the TA-26.
And, I haven't tested IEMs, or mid-range 120ohm-300ohm headphones on the TA-26 yet. So, I've not completed testing/comparing the TA-26 vs TA-20. The TA-26 has that 60ohm low-end impedance range. IEM's 60ohms and under - most of them(?) may not sound as good on the TA-26 as on the TA-20, similar to what I heard with the DT770 16 ohm headphones on the TA-26.
On the TA-20, after I solved EMI/RFI and PC/GPU noise issues with Mogami 2534 QuadStar cable built by WBC, and the iFi iDefender+ on the USB input to the D90. IEM's which in general have a lower range of impedance than headphones, as well as higher sensitivity to system noise levels, sound great to me on the TA-20 - with a noise-free black background. The TA-26 has a noise-free quiet background on my headphones tested so far, but I've not tried sensitive headphones/IEM's on the TA-26 yet.
The TA-20 in either Bal/SE does great for all of my lower impedance IEM's and headphones, on up to 600 ohm, where most amps start to "fade" in power to drive 600 ohm headphones, the TA-20 drives headphones across the entire impedance range. As compared to the TA-20, the TA-26 has more "power/drive" on my 600 ohm DT880SE - it sounds "glorious", very tubey - even with the stock TA-26 tubes.
The TA-20 Hybrid Tube/SS design does a great job with IEM's, dynamic, BA, piezo, planar, hybrid designs, and also sounds great with dynamic, and planar headphones. They all sound great, and I've not been disappointed with anything I've listened with on the TA-20.
Also, that might mean changing other components and their values, which is getting out there as far as potential benefit goes (to my knowledge).