Short answer
Both are dongles, $66 against $79.
The FiiO KA3 reaches 100 dB on 565 of the 567 models here, the ddHiFi TC35Pro (Mountain2) on 554.
| Specification | ddHiFi TC35Pro (Mountain2) | FiiO KA3 |
|---|---|---|
| Power into 32Ω | 65 mW | 130 mW |
| Power into 150Ω | 14 mW | 28 mW |
| Power into 300Ω | 6.9 mW | 14 mW |
| Power into 600Ω | 3.5 mW | 7.0 mW |
| Output impedance | not published | 0.5 Ω |
| Output noise | not published | 1.7 µV |
| Price | $66 | $79 |
| Headphones it reaches 100 dB on out of 567 | 554 | 565 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | ddHiFi TC35Pro (Mountain2) | FiiO KA3 |
|---|---|---|
| Socket | no separate rating | 4.4mm |
| Power into 32Ω | — | 240 mW |
| Power into 300Ω | — | 56 mW |
| Output impedance | — | 0.7 Ω |
| Output noise | — | 2.9 µV |
| Headphones it reaches 100 dB on out of 567 | — | 565 |
Only the FiiO KA3 has a separate balanced rating from its maker; through it, it reaches 100 dB on 565 of the 567 models here. The ddHiFi TC35Pro (Mountain2) may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it.
| Load | ddHiFi TC35Pro (Mountain2) | FiiO KA3 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Published |
| 600 Ω | Derived | Published |
Both makers publish a power rating. ddHiFi TC35Pro (Mountain2) publishes at 32 Ω; FiiO KA3 at 32 Ω, at 300 Ω and at 600 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω and at 600 Ω only the FiiO KA3 figure is the maker’s own. On those rows the derived number is the one to doubt first.
11 headphones reach 100 dB on the FiiO KA3 but not on the ddHiFi TC35Pro (Mountain2) — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 8 more.
554 of the 567 models here reach 100 dB on both.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the Campfire Audio Andromeda 2020: FiiO KA3 has +12 dB of headroom there against +7 dB. 6 dB is about 3.7 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for ddHiFi TC35Pro (Mountain2) because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | ddHiFi TC35Pro (Mountain2) headroom | FiiO KA3 headroom | ddHiFi TC35Pro (Mountain2) hiss | FiiO KA3 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +27 dB | +32 dB | not published | 19 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +7 dB | +12 dB | not published | -1 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +16 dB | +19 dB | not published | -3 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +11 dB | +14 dB | not published | -7 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +8 dB | +12 dB | not published | -8 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +4 dB | +8 dB | not published | -14 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +0 dB | +3 dB | not published | -18 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | -2 dB | +1 dB | not published | -20 dB |
The FiiO KA3 costs $13 more than the ddHiFi TC35Pro (Mountain2). Out of the 567 models here, that difference buys 11 more models driven to 100 dB.
Nothing runs the other way: there is no axis here on which the cheaper of the two does better.
Only published figures count here; blank axes are counted separately.
Each in full: ddHiFi TC35Pro (Mountain2) and FiiO KA3.