Short answer
Both are desktop amplifiers, $375 against $249.
The Topping A30 Pro reaches 100 dB on 567 of the 567 models here, the Bottlehead Crack (OTL kit) on 565.
| Specification | Bottlehead Crack (OTL kit) | Topping A30 Pro |
|---|---|---|
| Power into 32Ω | 271 mW | 5.5 W |
| Power into 150Ω | 403 mW | 1.4 W |
| Power into 300Ω | 333 mW | 840 mW |
| Power into 600Ω | 227 mW | 420 mW |
| Output impedance | 120.0 Ω | 0.1 Ω |
| Output noise | not published | 0.3 µV |
| Price | $375 | $249 |
| 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) | Topping A30 Pro |
|---|---|---|
| 32 Ω | Derived | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Published | Published |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. Bottlehead Crack (OTL kit) publishes at 300 Ω; Topping A30 Pro at 32 Ω and at 300 Ω. The other rows are worked out from those.
So the two columns are not equally firm row by row: at 32 Ω only the Topping A30 Pro 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 Topping A30 Pro but not on the Bottlehead Crack (OTL kit) — Moondrop PARA, Symphonium Titan.
567 headphones are left alone by the Topping A30 Pro 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: Topping A30 Pro 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 | Topping A30 Pro headroom | Bottlehead Crack (OTL kit) hiss | Topping A30 Pro hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +33 dB damping | +49 dB | not published | 4 dB |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +13 dB damping | +29 dB | not published | -16 dB |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +26 dB damping | +35 dB | not published | -18 dB |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +27 dB damping | +32 dB | not published | -22 dB |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB damping | +28 dB | not published | -23 dB |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +22 dB damping | +25 dB | not published | -29 dB |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +17 dB damping | +21 dB | not published | -33 dB |
| HiFiMAN Susvara | 60 Ω | 50 mW | +7 dB damping | +18 dB | not published | -35 dB |
The Bottlehead Crack (OTL kit) costs $126 more than the Topping A30 Pro, 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 Topping A30 Pro does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper Topping A30 Pro 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 Topping A30 Pro.