Short answer
Both are desktop amplifiers, $150 against $109.
The Schiit Magni+ reaches 100 dB on 567 of the 567 models here, the PreSonus HP4 on 558.
| Specification | PreSonus HP4 | Schiit Magni+ |
|---|---|---|
| Power into 32Ω | 69 mW | 2.0 W |
| Power into 150Ω | 52 mW | 560 mW |
| Power into 300Ω | 26 mW | 260 mW |
| Power into 600Ω | 13 mW | 130 mW |
| Output impedance | not published | 0.2 Ω unverified |
| Output noise | not published | 1.5 µV unverified |
| Price | $150 | $109 |
| Headphones it reaches 100 dB on out of 567 | 558 | 567 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | PreSonus HP4 | Schiit Magni+ |
|---|---|---|
| 32 Ω | Derived | Unverified |
| 150 Ω | Derived | Unverified |
| 300 Ω | Derived | Unverified |
| 600 Ω | Derived | Unverified |
Schiit Magni+ publishes no electrical specification at all. That is true of 10 of the 280 sources here, and the figures in that column come from independent measurement rather than from a maker document — which is not worse, but it is not the same thing, and it cannot be checked against anything the maker will stand behind.
The PreSonus HP4 column stands on figures its maker printed, but not at these loads — every row of it is worked out from power published somewhere else. Reading the two straight across sets one kind of claim against another, and nobody has written down the conditions of both side by side.
9 headphones reach 100 dB on the Schiit Magni+ but not on the PreSonus HP4 — Moondrop COSMO, Moondrop PARA, Moondrop PARA 2, 6 more.
558 of the 567 models here reach 100 dB on both.
Headroom is at a 100 dB SPL peak.
The widest gap on the list is the HiFiMAN HE400se: Schiit Magni+ has +23 dB of headroom there against +8 dB. 15 dB is about 29 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for PreSonus HP4 and Schiit Magni+ because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | PreSonus HP4 headroom | Schiit Magni+ headroom | PreSonus HP4 hiss | Schiit Magni+ hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +27 dB | +42 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +7 dB | +22 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +21 dB | +31 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +17 dB | +27 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +8 dB | +23 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +20 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +6 dB | +16 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +4 dB | +14 dB | not published | not published |
The PreSonus HP4 costs $41 more than the Schiit Magni+, and on these three axes the money buys nothing: among the 567 models here there is not one that the PreSonus HP4 handles and the Schiit Magni+ does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper Schiit Magni+ drives 9 models to 100 dB that the PreSonus HP4 does not.
Only published figures count here; blank axes are counted separately.
Each has a page of its own with a full headphone list and the provenance of every figure — worth opening here, because Schiit Magni+ has published no electrical specification at all and everything above comes from independent measurement: PreSonus HP4 and Schiit Magni+.