Short answer
Both are desktop amplifiers, $375 against $539.
The SMSL H400 MK2 reaches 100 dB on 567 of the 567 models here, the Bottlehead Crack (OTL kit) on 565.
| Specification | Bottlehead Crack (OTL kit) | SMSL H400 MK2 |
|---|---|---|
| Power into 32Ω | 271 mW | 2.5 W |
| Power into 150Ω | 403 mW | 527 mW |
| Power into 300Ω | 333 mW | 260 mW |
| Power into 600Ω | 227 mW | 130 mW |
| Output impedance | 120.0 Ω | <0.1 Ω |
| Output noise | not published | not published |
| Price | $375 | $539 |
| Headphones it reaches 100 dB on out of 567 | 565 | 567 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Bottlehead Crack (OTL kit) | SMSL H400 MK2 |
|---|---|---|
| Socket | no separate rating | 4.4mm / 4-pin XLR |
| Power into 32Ω | — | 9.0 W |
| Power into 300Ω | — | 1.1 W |
| Output impedance | — | <0.1 Ω |
| Output noise | — | not published |
| Headphones it reaches 100 dB on out of 567 | — | 567 |
Only the SMSL H400 MK2 has a separate balanced rating from its maker; through it, it reaches 100 dB on 567 of the 567 models here. The Bottlehead Crack (OTL kit) may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it.
| Load | Bottlehead Crack (OTL kit) | SMSL H400 MK2 |
|---|---|---|
| 32 Ω | Derived | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Published |
Both makers publish a power rating. Bottlehead Crack (OTL kit) publishes at 300 Ω; SMSL H400 MK2 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 32 Ω and at 600 Ω only the SMSL H400 MK2 figure is the maker’s own. On those rows the derived number is the one to doubt first.
2 headphones reach 100 dB on the SMSL H400 MK2 but not on the Bottlehead Crack (OTL kit) — Moondrop PARA, Symphonium Titan.
567 headphones are left alone by the SMSL H400 MK2 under the one-eighth rule but coloured by the Bottlehead Crack (OTL kit) (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 564 more).
565 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: SMSL H400 MK2 has +28 dB of headroom there against +13 dB. 16 dB is about 37 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Bottlehead Crack (OTL kit) and SMSL H400 MK2 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Bottlehead Crack (OTL kit) headroom | SMSL H400 MK2 headroom | Bottlehead Crack (OTL kit) hiss | SMSL H400 MK2 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +33 dB damping | +48 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +13 dB damping | +28 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB damping | +32 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +27 dB damping | +27 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB damping | +26 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +22 dB damping | +20 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +17 dB damping | +16 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +7 dB damping | +14 dB | not published | not published |
The SMSL H400 MK2 costs $164 more than the Bottlehead Crack (OTL kit). Out of the 567 models here, that difference buys 2 more models driven to 100 dB and 567 more models left alone under the one-eighth rule.
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: Bottlehead Crack (OTL kit) and SMSL H400 MK2.