RME ADI-2 DAC FS vs RME Fireface UCX II

Short answer

More power: the RME ADI-2 DAC FS, with 1.20 W into 32 Ω; the RME Fireface UCX II has 210 mW. The RME ADI-2 DAC FS drives 3 headphones to 100 dB that the RME Fireface UCX II cannot (Moondrop PARA, Symphonium Crimson, Symphonium Titan).
Hiss with sensitive IEMs: cannot be compared — the maker of the RME Fireface UCX II publishes no noise figure.
Output impedance: 15 headphones are left alone by the RME ADI-2 DAC FS under the one-eighth rule but coloured by the RME Fireface UCX II (64 Audio Nio, 64 Audio Volur, Campfire Audio Andromeda 2023, 12 more).
Price: the RME ADI-2 DAC FS costs $350 less, and wherever both makers publish a figure the RME Fireface UCX II shows no advantage for the extra money.

Both are audio interfaces, $1399 against $1749.

The RME ADI-2 DAC FS reaches 100 dB on 567 of the 567 models here, the RME Fireface UCX II on 564.

Every published figure, next to each other

Specification	RME ADI-2 DAC FS	RME Fireface UCX II
Power into 32Ω	1.2 W	210 mW
Power into 150Ω	256 mW	45 mW
Power into 300Ω	128 mW	22 mW
Power into 600Ω	64 mW	11 mW
Output impedance	0.1 Ω	1 Ω
Output noise	2.18 µV	not published
Price	$1399	$1749
Headphones it reaches 100 dB on out of 567	567	564

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

Where each power figure comes from

Load	RME ADI-2 DAC FS	RME Fireface UCX II
32 Ω	Published	Published
150 Ω	Derived	Derived
300 Ω	Derived	Derived
600 Ω	Derived	Derived

Both makers publish a power rating. RME ADI-2 DAC FS publishes at 32 Ω; RME Fireface UCX II at 32 Ω. The other rows are worked out from those.

The two publish at the same loads, which is the one case where the rows of this table can be read straight across.

What the labels mean.

The headphones that tell them apart

3 headphones reach 100 dB on the RME ADI-2 DAC FS but not on the RME Fireface UCX II — Moondrop PARA, Symphonium Crimson, Symphonium Titan.

15 headphones are left alone by the RME ADI-2 DAC FS under the one-eighth rule but coloured by the RME Fireface UCX II (64 Audio Nio, 64 Audio Volur, Campfire Audio Andromeda 2023, 12 more).

564 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: RME ADI-2 DAC FS has +41 dB of headroom there against +32 dB. 10 dB is about 9 times the power, which matters for peaks rather than for how loud the music sits.

The hiss columns are empty for RME Fireface UCX II because that maker publishes no output noise figure.

Headphone	Impedance	Needs	RME ADI-2 DAC FS headroom	RME Fireface UCX II headroom	RME ADI-2 DAC FS hiss	RME Fireface UCX II hiss
Shure SE846	9 Ω	0.04 mW	+41 dB	+32 dB	21 dB	not published
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+21 dB	+12 dB	1 dB	not published
Audeze LCD-2 Classic	70 Ω	0.79 mW	+28 dB	+21 dB	-1 dB	not published
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+24 dB	+16 dB	-5 dB	not published
HiFiMAN HE400se	25 Ω	7.9 mW	+21 dB	+13 dB	-6 dB	not published
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+17 dB	+10 dB	-12 dB	not published
Sennheiser HD 600	300 Ω	6.6 mW	+13 dB	+5 dB	-16 dB	not published
HiFiMAN Susvara	60 Ω	50 mW	+11 dB	+3 dB	-18 dB	not published

The gap in dollars, against the gap in numbers

The RME Fireface UCX II costs $350 more than the RME ADI-2 DAC FS, and on these three axes the money buys nothing: among the 567 models here there is not one that the RME Fireface UCX II handles and the RME ADI-2 DAC FS does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper RME ADI-2 DAC FS drives 3 models to 100 dB and leaves 15 alone under the one-eighth rule that the RME Fireface UCX II does not.

Only published figures count here; blank axes are counted separately.