Apogee BOOM vs Focusrite Scarlett 4i4 4th gen

Short answer

More power: the Apogee BOOM, with 240 mW into 32 Ω; the Focusrite Scarlett 4i4 4th gen has 55 mW. The Apogee BOOM drives 12 headphones to 100 dB that the Focusrite Scarlett 4i4 4th gen cannot (HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 9 more).
Hiss with sensitive IEMs: cannot be compared — neither maker publishes a noise figure.
Output impedance: 519 headphones are left alone by the Apogee BOOM under the one-eighth rule but coloured by the Focusrite Scarlett 4i4 4th gen (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 516 more).
Price: the Apogee BOOM costs $20 more; what that buys on these numbers is set out further down, under What the price difference buys.

Both are audio interfaces, $299 against $279.

The Apogee BOOM reaches 100 dB on 565 of the 567 models here, the Focusrite Scarlett 4i4 4th gen on 553.

Every published figure, next to each other

Specification	Apogee BOOM	Focusrite Scarlett 4i4 4th gen
Power into 32Ω	240 mW	55 mW
Power into 150Ω	51 mW	27 mW
Power into 300Ω	26 mW	27 mW
Power into 600Ω	13 mW	14 mW
Output impedance	0.5 Ω	11 Ω
Output noise	not published	not published
Price	$299	$279
Headphones it reaches 100 dB on out of 567	565	553

Not published means the maker leaves it out, so that axis cannot be compared.

Where each power figure comes from

Load	Apogee BOOM	Focusrite Scarlett 4i4 4th gen
32 Ω	Published	Derived
150 Ω	Derived	Derived
300 Ω	Derived	Published
600 Ω	Derived	Derived

Both makers publish a power rating. Apogee BOOM publishes at 32 Ω; Focusrite Scarlett 4i4 4th gen 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 Apogee BOOM figure is the maker’s own and at 300 Ω only the Focusrite Scarlett 4i4 4th gen 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

12 headphones reach 100 dB on the Apogee BOOM but not on the Focusrite Scarlett 4i4 4th gen — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 9 more.

519 headphones are left alone by the Apogee BOOM under the one-eighth rule but coloured by the Focusrite Scarlett 4i4 4th gen (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Nio, 516 more).

553 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 HiFiMAN HE400se: Apogee BOOM has +14 dB of headroom there against +7 dB. 6 dB is about 4.3 times the power, which matters for peaks rather than for how loud the music sits.

The hiss columns are empty for Apogee BOOM and Focusrite Scarlett 4i4 4th gen because neither maker publishes no output noise figure.

Headphone	Impedance	Needs	Apogee BOOM headroom	Focusrite Scarlett 4i4 4th gen headroom	Apogee BOOM hiss	Focusrite Scarlett 4i4 4th gen hiss
Shure SE846	9 Ω	0.04 mW	+32 dB	+26 dB damping	not published	not published
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+12 dB	+6 dB damping	not published	not published
Audeze LCD-2 Classic	70 Ω	0.79 mW	+21 dB	+16 dB damping	not published	not published
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+17 dB	+16 dB	not published	not published
HiFiMAN HE400se	25 Ω	7.9 mW	+14 dB	+7 dB damping	not published	not published
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+10 dB	+10 dB	not published	not published
Sennheiser HD 600	300 Ω	6.6 mW	+6 dB	+6 dB	not published	not published
HiFiMAN Susvara	60 Ω	50 mW	+4 dB	-1 dB damping	not published	not published

The gap in dollars, against the gap in numbers

The Apogee BOOM costs $20 more than the Focusrite Scarlett 4i4 4th gen. Out of the 567 models here, that difference buys 12 more models driven to 100 dB and 519 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.