Bottlehead Crack (OTL kit) vs SMSL SH-9

Short answer

More power: the SMSL SH-9, with 3.00 W into 32 Ω; the Bottlehead Crack (OTL kit) has 271 mW. The SMSL SH-9 drives 2 headphones to 100 dB that the Bottlehead Crack (OTL kit) cannot (Moondrop PARA, Symphonium Titan).
Hiss with sensitive IEMs: cannot be compared — the maker of the Bottlehead Crack (OTL kit) publishes no noise figure.
Output impedance: 567 headphones are left alone by the SMSL SH-9 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).
Price: the SMSL SH-9 costs $86 less, and wherever both makers publish a figure the Bottlehead Crack (OTL kit) shows no advantage for the extra money.

Both are desktop amplifiers, $375 against $289.

The SMSL SH-9 reaches 100 dB on 567 of the 567 models here, the Bottlehead Crack (OTL kit) on 565.

Every published figure, next to each other

Specification	Bottlehead Crack (OTL kit)	SMSL SH-9
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	1.5 µV unverified
Price	$375	$289
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.

Where each power figure comes from

Load	Bottlehead Crack (OTL kit)	SMSL SH-9
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 SH-9 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 SH-9 figure is the maker’s own. On those rows the derived number is the one to doubt first.

What the labels mean.

The headphones that tell them apart

2 headphones reach 100 dB on the SMSL SH-9 but not on the Bottlehead Crack (OTL kit) — Moondrop PARA, Symphonium Titan.

567 headphones are left alone by the SMSL SH-9 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 and hiss on 8 models, both sources

Headroom is at a 100 dB SPL peak.

The widest gap on the list is the Shure SE846: SMSL SH-9 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) and SMSL SH-9 because neither maker publishes no output noise figure.

Headphone	Impedance	Needs	Bottlehead Crack (OTL kit) headroom	SMSL SH-9 headroom	Bottlehead Crack (OTL kit) hiss	SMSL SH-9 hiss
Shure SE846	9 Ω	0.04 mW	+33 dB damping	+49 dB	not published	not published
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+13 dB damping	+29 dB	not published	not published
Audeze LCD-2 Classic	70 Ω	0.79 mW	+26 dB damping	+33 dB	not published	not published
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+27 dB damping	+29 dB	not published	not published
HiFiMAN HE400se	25 Ω	7.9 mW	+14 dB damping	+27 dB	not published	not published
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+22 dB damping	+22 dB	not published	not published
Sennheiser HD 600	300 Ω	6.6 mW	+17 dB damping	+18 dB	not published	not published
HiFiMAN Susvara	60 Ω	50 mW	+7 dB damping	+15 dB	not published	not published

The gap in dollars, against the gap in numbers

The Bottlehead Crack (OTL kit) costs $86 more than the SMSL SH-9, 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 SH-9 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper SMSL SH-9 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.