Short answer
Both are audio interfaces, $229 against $249.
The SSL 2 MkII reaches 100 dB on 565 of the 567 models here, the Tascam US-4x4HR on 553.
| Specification | SSL 2 MkII | Tascam US-4x4HR |
|---|---|---|
| Power into 32Ω | 374 mW | 45 mW |
| Power into 150Ω | 80 mW | 36 mW |
| Power into 300Ω | 40 mW | 24 mW |
| Power into 600Ω | 20 mW | 14 mW |
| Output impedance | 1 Ω | 51.5 Ω |
| Output noise | not published | not published |
| Price | $229 | $249 |
| Headphones it reaches 100 dB on out of 567 | 565 | 553 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Load | SSL 2 MkII | Tascam US-4x4HR |
|---|---|---|
| 32 Ω | Upper bound | Published |
| 150 Ω | Upper bound | Derived |
| 300 Ω | Upper bound | Derived |
| 600 Ω | Upper bound | Derived |
These two columns are not the same kind of number, and that matters more than the gap between them. Tascam US-4x4HR publishes a power rating (at 32 Ω); SSL 2 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 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 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.
12 headphones reach 100 dB on the SSL 2 MkII but not on the Tascam US-4x4HR — HiFiMAN HE-6se V2, HiFiMAN HE6 Remastered, HiFiMAN Susvara, 9 more.
546 headphones are left alone by the SSL 2 MkII under the one-eighth rule but coloured by the Tascam US-4x4HR (64 Audio Aspire 1, 64 Audio Fourte, 64 Audio Ocean, 543 more).
553 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 Shure SE846: SSL 2 MkII has +34 dB of headroom there against +25 dB. 9 dB is about 8.3 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for SSL 2 MkII and Tascam US-4x4HR because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | SSL 2 MkII headroom | Tascam US-4x4HR headroom | SSL 2 MkII hiss | Tascam US-4x4HR hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +34 dB | +25 dB damping | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +14 dB | +5 dB damping | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +23 dB | +16 dB damping | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +19 dB | +16 dB damping | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +16 dB | +6 dB damping | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +12 dB | +10 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +8 dB | +6 dB damping | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +6 dB | -1 dB damping | not published | not published |
The Tascam US-4x4HR costs $20 more than the SSL 2 MkII, and on these three axes the money buys nothing: among the 567 models here there is not one that the Tascam US-4x4HR handles and the SSL 2 MkII does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper SSL 2 MkII drives 12 models to 100 dB and leaves 546 alone under the one-eighth rule that the Tascam US-4x4HR does not.
Only published figures count here; blank axes are counted separately.
Each in full: SSL 2 MkII and Tascam US-4x4HR.