Short answer
Both are desktop amplifiers, $375 against $529.
The SMSL SP400 reaches 100 dB on 567 of the 567 models here, the Bottlehead Crack (OTL kit) on 565.
| Specification | Bottlehead Crack (OTL kit) | SMSL SP400 |
|---|---|---|
| Power into 32Ω | 271 mW | 3.0 W |
| Power into 150Ω | 403 mW | 741 mW |
| Power into 300Ω | 333 mW | 440 mW |
| Power into 600Ω | 227 mW | 220 mW |
| Output impedance | 120.0 Ω | <0.1 Ω |
| Output noise | not published | 1.9 µV |
| Price | $375 | $529 |
| 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 SP400 |
|---|---|---|
| Socket | no separate rating | XLR4 |
| Power into 32Ω | — | 6.0 W |
| Power into 300Ω | — | 880 mW |
| Output impedance | — | <0.1 Ω |
| Output noise | — | 1.9 µV |
| Headphones it reaches 100 dB on out of 567 | — | 567 |
Only the SMSL SP400 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 SP400 |
|---|---|---|
| 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 SP400 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 SP400 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 SP400 but not on the Bottlehead Crack (OTL kit) — Moondrop PARA, Symphonium Titan.
567 headphones are left alone by the SMSL SP400 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 Shure SE846: SMSL SP400 has +49 dB of headroom there against +33 dB. 16 dB is about 44 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) because that maker publishes no output noise figure.
| Headphone | Impedance | Needs | Bottlehead Crack (OTL kit) headroom | SMSL SP400 headroom | Bottlehead Crack (OTL kit) hiss | SMSL SP400 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +33 dB damping | +49 dB | not published | 20 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +13 dB damping | +29 dB | not published | 0 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB damping | +33 dB | not published | -2 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +27 dB damping | +29 dB | not published | -6 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB damping | +27 dB | not published | -7 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +22 dB damping | +22 dB | not published | -13 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +17 dB damping | +18 dB | not published | -17 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +7 dB damping | +15 dB | not published | -19 dB |
The SMSL SP400 costs $154 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 SP400.