Short answer
Both are desktop amplifiers, $375 against $296.
The SMSL H300 reaches 100 dB on 567 of the 567 models here, the Bottlehead Crack (OTL kit) on 565.
| Specification | Bottlehead Crack (OTL kit) | SMSL H300 |
|---|---|---|
| Power into 32Ω | 271 mW | 5.0 W |
| Power into 150Ω | 403 mW | 1.1 W |
| Power into 300Ω | 333 mW | 533 mW |
| Power into 600Ω | 227 mW | 267 mW |
| Output impedance | 120.0 Ω | <0.1 Ω |
| Output noise | not published | not published |
| Price | $375 | $296 |
| 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.
| Load | Bottlehead Crack (OTL kit) | SMSL H300 |
|---|---|---|
| 32 Ω | Derived | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. Bottlehead Crack (OTL kit) publishes at 300 Ω; SMSL H300 at 32 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 300 Ω only the Bottlehead Crack (OTL kit) figure is the maker’s own and at 32 Ω only the SMSL H300 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 H300 but not on the Bottlehead Crack (OTL kit) — Moondrop PARA, Symphonium Titan.
567 headphones are left alone by the SMSL H300 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 H300 has +32 dB of headroom there against +13 dB. 19 dB is about 74 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 H300 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Bottlehead Crack (OTL kit) headroom | SMSL H300 headroom | Bottlehead Crack (OTL kit) hiss | SMSL H300 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +33 dB damping | +52 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +13 dB damping | +32 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB damping | +35 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +27 dB damping | +30 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB damping | +29 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +22 dB damping | +23 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +17 dB damping | +19 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +7 dB damping | +17 dB | not published | not published |
The Bottlehead Crack (OTL kit) costs $79 more than the SMSL H300, and on these three axes the money buys nothing: among the 567 models here there is not one that the Bottlehead Crack (OTL kit) handles and the SMSL H300 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper SMSL H300 drives 2 models to 100 dB and leaves 567 alone under the one-eighth rule that the Bottlehead Crack (OTL kit) does not.
Only published figures count here; blank axes are counted separately.
Each in full: Bottlehead Crack (OTL kit) and SMSL H300.