Short answer
Both are dongles, $499 against $449.
The Cayin RU9 reaches 100 dB on 563 of the 567 models here, the Luxury & Precision W4 on 561.
| Specification | Cayin RU9 | Luxury & Precision W4 |
|---|---|---|
| Power into 32Ω | 160 mW | 110 mW |
| Power into 150Ω | 34 mW | 23 mW |
| Power into 300Ω | 17 mW | 12 mW |
| Power into 600Ω | 8.5 mW | 5.9 mW |
| Output impedance | 0.5 Ω | not published |
| Output noise | not published | not published |
| Price | $499 | $449 |
| Headphones it reaches 100 dB on out of 567 | 563 | 561 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Cayin RU9 | Luxury & Precision W4 |
|---|---|---|
| Socket | 4.4mm | 4.4mm |
| Power into 32Ω | 400 mW | 420 mW |
| Power into 300Ω | 43 mW derived | 45 mW derived |
| Output impedance | 1.0 Ω | not published |
| Output noise | not published | not published |
| Headphones it reaches 100 dB on out of 567 | 565 | 566 |
Through the balanced sockets the Luxury & Precision W4 reaches 100 dB on 566 of the 567 models here and the Cayin RU9 on 565. Derived figures are worked out from the loads the maker did publish.
| Load | Cayin RU9 | Luxury & Precision W4 |
|---|---|---|
| 32 Ω | Published | Published |
| 150 Ω | Derived | Derived |
| 300 Ω | Derived | Derived |
| 600 Ω | Derived | Derived |
Both makers publish a power rating. Cayin RU9 publishes at 32 Ω; Luxury & Precision W4 at 32 Ω. The other rows are worked out from those.
The two publish at the same loads, which is the one case where the rows of this table can be read straight across.
2 headphones reach 100 dB on the Cayin RU9 but not on the Luxury & Precision W4 — Moondrop COSMO, Symphonium Zenith.
561 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 Sennheiser HD 600: Cayin RU9 has +4 dB of headroom there against +2 dB. 2 dB is about 1.5 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Cayin RU9 and Luxury & Precision W4 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Cayin RU9 headroom | Luxury & Precision W4 headroom | Cayin RU9 hiss | Luxury & Precision W4 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +31 dB | +29 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +11 dB | +9 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +20 dB | +18 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +15 dB | +13 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +12 dB | +10 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +8 dB | +7 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +4 dB | +2 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +2 dB | +1 dB | not published | not published |
The Cayin RU9 costs $50 more than the Luxury & Precision W4. Out of the 567 models here, that difference buys 2 more models driven to 100 dB.
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.
Each in full: Cayin RU9 and Luxury & Precision W4.