Short answer
Both are audio interfaces, both at $299.
On raw power they are level: each reaches 100 dB on 565 of the 567 non-electrostatic models here.
| Specification | Apogee BOOM | SSL 2 Plus MkII |
|---|---|---|
| Power into 32Ω | 240 mW | 374 mW |
| Power into 150Ω | 51 mW | 80 mW |
| Power into 300Ω | 26 mW | 40 mW |
| Power into 600Ω | 13 mW | 20 mW |
| Output impedance | 0.5 Ω | 1 Ω |
| Output noise | not published | not published |
| Price | $299 | $299 |
| Headphones it reaches 100 dB on out of 567 | 565 | 565 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | Apogee BOOM | SSL 2 Plus MkII |
|---|---|---|
| 32 Ω | Published | Upper bound |
| 150 Ω | Derived | Upper bound |
| 300 Ω | Derived | Upper bound |
| 600 Ω | Derived | Upper bound |
These two columns are not the same kind of number, and that matters more than the gap between them. Apogee BOOM publishes a power rating (at 32 Ω); SSL 2 Plus MkII publishes no power figure at any load, only a maximum output voltage, so every milliwatt in its column is a ceiling worked out from that voltage.
A ceiling assumes the amplifier holds that voltage into the load, and nothing on SSL 2 Plus MkII’s own sheet says it does. It is the most flattering reading available, and it is used here only because the alternative is a blank. Where the SSL 2 Plus MkII column looks the stronger of the two, treat the difference as unproven rather than measured. 24 of the 280 sources here are in that position, with no published power rating at any load.
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.
14 headphones are left alone by the Apogee BOOM under the one-eighth rule but coloured by the SSL 2 Plus MkII (64 Audio Nio, 64 Audio Volur, Campfire Audio Andromeda 2023, 11 more).
565 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 Audio-Technica ATH-R70X: SSL 2 Plus MkII has +12 dB of headroom there against +10 dB. 2 dB is about 1.6 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Apogee BOOM and SSL 2 Plus MkII because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Apogee BOOM headroom | SSL 2 Plus MkII headroom | Apogee BOOM hiss | SSL 2 Plus MkII hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +32 dB | +34 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +12 dB | +14 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +21 dB | +23 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +17 dB | +19 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +14 dB | +16 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +10 dB | +12 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +6 dB | +8 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +4 dB | +6 dB | not published | not published |
These two cost the same, $299, so price settles nothing. Out of the 567 models here, the Apogee BOOM leaves 14 models alone under the one-eighth rule — and on the other side there is no such advantage at all.
Only published figures count here; blank axes are counted separately.
Each in full: Apogee BOOM and SSL 2 Plus MkII.