Audient iD48 vs RME Fireface UCX II

Short answer

More power: the Audient iD48, with 528 mW into 32 Ω; the RME Fireface UCX II has 210 mW. The Audient iD48 drives 2 headphones to 100 dB that the RME Fireface UCX II cannot (Moondrop PARA, Symphonium Crimson).
Hiss with sensitive IEMs: cannot be compared — neither maker publishes a noise figure.
Output impedance: 546 headphones are left alone by the RME Fireface UCX II under the one-eighth rule but coloured by the Audient iD48 (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 543 more).
Price: the RME Fireface UCX II costs $449 more; what that buys on these numbers is set out further down, under What the price difference buys.

Both are audio interfaces, $1300 against $1749.

The Audient iD48 reaches 100 dB on 566 of the 567 models here, the RME Fireface UCX II on 564.

Every published figure, next to each other

Specification	Audient iD48	RME Fireface UCX II
Power into 32Ω	528 mW	210 mW
Power into 150Ω	265 mW	45 mW
Power into 300Ω	168 mW	22 mW
Power into 600Ω	126 mW	11 mW
Output impedance	50 Ω	1 Ω
Output noise	not published	not published
Price	$1300	$1749
Headphones it reaches 100 dB on out of 567	566	564

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

Where each power figure comes from

Load	Audient iD48	RME Fireface UCX II
32 Ω	Derived	Published
150 Ω	Derived	Derived
300 Ω	Derived	Derived
600 Ω	Published	Derived

Both makers publish a power rating. Audient iD48 publishes at 600 Ω; RME Fireface UCX II at 32 Ω. The other rows are worked out from those.

So the two columns are not equally firm row by row: at 600 Ω only the Audient iD48 figure is the maker’s own and at 32 Ω only the RME Fireface UCX II 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 Audient iD48 but not on the RME Fireface UCX II — Moondrop PARA, Symphonium Crimson.

546 headphones are left alone by the RME Fireface UCX II under the one-eighth rule but coloured by the Audient iD48 (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 543 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 Audio-Technica ATH-R70X: Audient iD48 has +19 dB of headroom there against +10 dB. 10 dB is about 9.5 times the power, which matters for peaks rather than for how loud the music sits.

The hiss columns are empty for Audient iD48 and RME Fireface UCX II because neither maker publishes no output noise figure.

Headphone	Impedance	Needs	Audient iD48 headroom	RME Fireface UCX II headroom	Audient iD48 hiss	RME Fireface UCX II hiss
Shure SE846	9 Ω	0.04 mW	+36 dB damping	+32 dB	not published	not published
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+16 dB damping	+12 dB	not published	not published
Audeze LCD-2 Classic	70 Ω	0.79 mW	+28 dB damping	+21 dB	not published	not published
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+25 dB damping	+16 dB	not published	not published
HiFiMAN HE400se	25 Ω	7.9 mW	+17 dB damping	+13 dB	not published	not published
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+19 dB	+10 dB	not published	not published
Sennheiser HD 600	300 Ω	6.6 mW	+14 dB damping	+5 dB	not published	not published
HiFiMAN Susvara	60 Ω	50 mW	+11 dB damping	+3 dB	not published	not published

The gap in dollars, against the gap in numbers

The RME Fireface UCX II costs $449 more than the Audient iD48. Out of the 567 models here, that difference buys 546 more models left alone under the one-eighth rule.

It does not run one way only: the cheaper Audient iD48 drives 2 models to 100 dB that the RME Fireface UCX II does not, which is the part worth checking against your own headphones before paying the difference.

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