Benchmark HPA4 vs Violectric V550 Pro

Short answer

More power: the Violectric V550 Pro, with 4.70 W into 32 Ω; the Benchmark HPA4 has 3.35 W (balanced: 4.70 W against 3.35 W). Both drive 567 of the 567 non-electrostatic headphones here to 100 dB, so on that test the difference changes nothing.
Quieter with sensitive IEMs: the Benchmark HPA4. 65 headphones stay below audible hiss on it but not on the Violectric V550 Pro (64 Audio Nio, 64 Audio U12t, 64 Audio U18s, 62 more).
Price: the Benchmark HPA4 costs $101 less and gives up nothing on power, output impedance or hiss against the 567 headphones here. Whatever the extra money for the Violectric V550 Pro buys, it is not these three numbers.

Both are desktop amplifiers, $3499 against $3600.

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

Specification	Benchmark HPA4	Violectric V550 Pro
Power into 32Ω	3.4 W	4.7 W
Power into 150Ω	829 mW	2.9 W
Power into 300Ω	440 mW	1.4 W
Power into 600Ω	220 mW	740 mW
Output impedance	<0.1 Ω	0.15 Ω
Output noise	2.45 µV	5.48 µV
Price	$3499	$3600
Headphones it reaches 100 dB on out of 567	567	567

Specification

Benchmark HPA4

Violectric V550 Pro

Power into 32Ω

3.4 W

4.7 W

Power into 150Ω

829 mW

2.9 W

Power into 300Ω

440 mW

1.4 W

Power into 600Ω

220 mW

740 mW

Output impedance

<0.1 Ω

0.15 Ω

Output noise

2.45 µV

5.48 µV

Price

$3499

$3600

Headphones it reaches 100 dB on
out of 567

567

The 4.4 mm or XLR socket, rated on its own

Balanced	Benchmark HPA4	Violectric V550 Pro
Socket	balanced	balanced
Power into 32Ω	3.4 W derived	4.7 W
Power into 300Ω	440 mW	1.4 W
Output impedance	<0.1 Ω	0.3 Ω
Output noise	2.45 µV	5.48 µV
Headphones it reaches 100 dB on out of 567	567	567

Through the balanced sockets the two are level on the same test: each reaches 100 dB on 567 of the 567 models here. Derived figures are worked out from the loads the maker did publish.

Where each power figure comes from

Load	Benchmark HPA4	Violectric V550 Pro
32 Ω	Derived	Published
150 Ω	Derived	Derived
300 Ω	Published	Published
600 Ω	Derived	Published

Both makers publish a power rating. Benchmark HPA4 publishes at 300 Ω; Violectric V550 Pro 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 Violectric V550 Pro 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.

65 headphones stay below the audible-hiss threshold on the Benchmark HPA4 but cross it on the Violectric V550 Pro (64 Audio Nio, 64 Audio U12t, 64 Audio U18s, 62 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 Beyerdynamic DT 1990 Pro (250Ω): Violectric V550 Pro has +34 dB of headroom there against +29 dB. 5 dB is about 3.3 times the power, which matters for peaks rather than for how loud the music sits.

Hiss below about 20 dB SPL is inaudible in a quiet room.

Headphone	Impedance	Needs	Benchmark HPA4 headroom	Violectric V550 Pro headroom	Benchmark HPA4 hiss	Violectric V550 Pro hiss
Shure SE846	9 Ω	0.04 mW	+49 dB	+45 dB	22 dB	29 dB
Campfire Audio Andromeda 2020	9 Ω	4.0 mW	+29 dB	+25 dB	2 dB	9 dB
Audeze LCD-2 Classic	70 Ω	0.79 mW	+33 dB	+38 dB	0 dB	7 dB
Beyerdynamic DT 1990 Pro (250Ω)	250 Ω	0.63 mW	+29 dB	+34 dB	-4 dB	3 dB
HiFiMAN HE400se	25 Ω	7.9 mW	+27 dB	+26 dB	-5 dB	2 dB
Audio-Technica ATH-R70X	470 Ω	1.6 mW	+22 dB	+28 dB	-11 dB	-4 dB
Sennheiser HD 600	300 Ω	6.6 mW	+18 dB	+23 dB	-15 dB	-8 dB
HiFiMAN Susvara	60 Ω	50 mW	+16 dB	+21 dB	-17 dB	-10 dB

The gap in dollars, against the gap in numbers

The Violectric V550 Pro costs $101 more than the Benchmark HPA4, and on these three axes the money buys nothing: among the 567 models here there is not one that the Violectric V550 Pro handles and the Benchmark HPA4 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper Benchmark HPA4 keeps 65 models below the audible-hiss threshold that the Violectric V550 Pro does not.

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