JDS Labs Element III MK2 vs SMSL DO400

Short answer

More power: the SMSL DO400, with 3.00 W into 32 Ω; the JDS Labs Element III MK2 has 2.30 W. Both drive 567 of the 567 non-electrostatic headphones here to 100 dB, so on that test the difference changes nothing.
Hiss with sensitive IEMs: cannot be compared — the maker of the SMSL DO400 publishes no noise figure.
Output impedance: 4 headphones are left alone by the SMSL DO400 under the one-eighth rule but coloured by the JDS Labs Element III MK2 (Campfire Audio Chimera, Moondrop Armature Art 12, Softears Volume, 1 more).
Price: the SMSL DO400 costs $71 more; what that buys on these numbers is set out further down, under What the price difference buys.

Both are desktop DAC/amps, $449 against $520.

On raw power they are level: each reaches 100 dB on 567 of the 567 non-electrostatic models here.

Every published figure, next to each other

Specification	JDS Labs Element III MK2	SMSL DO400
Power into 32Ω	2.3 W	3.0 W
Power into 150Ω	584 mW	938 mW
Power into 300Ω	292 mW	700 mW
Power into 600Ω	146 mW	350 mW
Output impedance	0.7 Ω	<0.1 Ω
Output noise	2.51 µV	not published
Price	$449	$520
Headphones it reaches 100 dB on out of 567	567	567

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

Where each power figure comes from

Load	JDS Labs Element III MK2	SMSL DO400
32 Ω	Published	Published
150 Ω	Published	Derived
300 Ω	Derived	Published
600 Ω	Published	Published

Both makers publish a power rating. JDS Labs Element III MK2 publishes at 32 Ω, at 150 Ω and at 600 Ω; SMSL DO400 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 150 Ω only the JDS Labs Element III MK2 figure is the maker’s own and at 300 Ω only the SMSL DO400 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

On power they are interchangeable across this whole database: there is no headphone here that one reaches 100 dB on and the other does not.

4 headphones are left alone by the SMSL DO400 under the one-eighth rule but coloured by the JDS Labs Element III MK2 (Campfire Audio Chimera, Moondrop Armature Art 12, Softears Volume, 1 more).

567 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 DO400 has +49 dB of headroom there against +42 dB. 7 dB is about 5.2 times the power, which matters for peaks rather than for how loud the music sits.

The hiss columns are empty for SMSL DO400 because that maker publishes no output noise figure.

Headphone	Impedance	Needs	JDS Labs Element III MK2 headroom	SMSL DO400 headroom	JDS Labs Element III MK2 hiss	SMSL DO400 hiss
Shure SE846	9 Ω	0.04 mW	+42 dB	+49 dB	22 dB	not published
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+22 dB	+29 dB	2 dB	not published
Audeze LCD-2 Classic	70 Ω	0.79 mW	+31 dB	+33 dB	1 dB	not published
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+27 dB	+31 dB	-4 dB	not published
HiFiMAN HE400se	25 Ω	7.9 mW	+24 dB	+27 dB	-5 dB	not published
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+21 dB	+25 dB	-11 dB	not published
Sennheiser HD 600	300 Ω	6.6 mW	+16 dB	+20 dB	-15 dB	not published
HiFiMAN Susvara	60 Ω	50 mW	+14 dB	+15 dB	-17 dB	not published

The gap in dollars, against the gap in numbers

The SMSL DO400 costs $71 more than the JDS Labs Element III MK2. Out of the 567 models here, that difference buys 4 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.