Short answer
Both are desktop DAC/amps, $799 against $520.
On raw power they are level: each reaches 100 dB on 567 of the 567 non-electrostatic models here.
| Specification | FiiO K9 Pro | SMSL DO400 |
|---|---|---|
| Power into 32Ω | 2.0 W | 3.0 W |
| Power into 150Ω | 482 mW | 938 mW |
| Power into 300Ω | 278 mW | 700 mW |
| Power into 600Ω | 139 mW | 350 mW |
| Output impedance | 1.0 Ω | <0.1 Ω |
| Output noise | 2.0 µV | not published |
| Price | $799 | $520 |
| Headphones it reaches 100 dB on out of 567 | 567 | 567 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | FiiO K9 Pro | SMSL DO400 |
|---|---|---|
| Socket | 4.4mm + XLR4 | no separate rating |
| Power into 32Ω | 2.0 W | — |
| Power into 300Ω | 1.1 W | — |
| Output impedance | 1.0 Ω | — |
| Output noise | 3.0 µV | — |
| Headphones it reaches 100 dB on out of 567 | 567 | — |
Only the FiiO K9 Pro has a separate balanced rating from its maker; through it, it reaches 100 dB on 567 of the 567 models here. The SMSL DO400 may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it.
| Load | FiiO K9 Pro | SMSL DO400 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Published |
Both makers publish a power rating. FiiO K9 Pro publishes at 32 Ω and at 300 Ω; SMSL DO400 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 600 Ω only the SMSL DO400 figure is the maker’s own. On those rows the derived number is the one to doubt first.
On power they are interchangeable across this whole database: there is no headphone here that one reaches 100 dB on and the other does not.
15 headphones are left alone by the SMSL DO400 under the one-eighth rule but coloured by the FiiO K9 Pro (64 Audio Nio, 64 Audio Volur, Campfire Audio Andromeda 2023, 12 more).
567 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 Shure SE846: SMSL DO400 has +49 dB of headroom there against +42 dB. 7 dB is about 5.5 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for SMSL DO400 because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | FiiO K9 Pro headroom | SMSL DO400 headroom | FiiO K9 Pro hiss | SMSL DO400 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +42 dB | +49 dB | 20 dB | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +22 dB | +29 dB | 0 dB | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +31 dB | +33 dB | -1 dB | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +27 dB | +31 dB | -6 dB | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +23 dB | +27 dB | -7 dB | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +20 dB | +25 dB | -13 dB | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +16 dB | +20 dB | -17 dB | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +13 dB | +15 dB | -19 dB | not published |
The FiiO K9 Pro costs $279 more than the SMSL DO400, and on these three axes the money buys nothing: among the 567 models here there is not one that the FiiO K9 Pro handles and the SMSL DO400 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper SMSL DO400 leaves 15 models alone under the one-eighth rule that the FiiO K9 Pro does not.
Only published figures count here; blank axes are counted separately.
Each in full: FiiO K9 Pro and SMSL DO400.