Short answer
Both are dongles, $269 against $250.
The Questyle M15 reaches 100 dB on 565 of the 567 models here, the Cayin RU7 on 563.
| Specification | Cayin RU7 | Questyle M15 |
|---|---|---|
| Power into 32Ω | 160 mW | 300 mW |
| Power into 150Ω | 34 mW | 110 mW |
| Power into 300Ω | 17 mW | 55 mW |
| Power into 600Ω | 8.5 mW | 28 mW |
| Output impedance | not published | 0.5 Ω unverified |
| Output noise | not published | 3 µV unverified |
| Price | $269 | $250 |
| Headphones it reaches 100 dB on out of 567 | 563 | 565 |
Not published means the maker leaves it out, so that axis cannot be compared.
| Balanced | Cayin RU7 | Questyle M15 |
|---|---|---|
| Socket | 4.4mm | no separate rating |
| Power into 32Ω | 400 mW | — |
| Power into 300Ω | 43 mW derived | — |
| Output impedance | not published | — |
| Output noise | not published | — |
| Headphones it reaches 100 dB on out of 567 | 565 | — |
Only the Cayin RU7 has a separate balanced rating from its maker; through it, it reaches 100 dB on 565 of the 567 models here. The Questyle M15 may still have a balanced socket — its spec sheet simply gives no figure for one, so there is nothing to put beside it. Derived figures are worked out from the loads the maker did publish.
| Load | Cayin RU7 | Questyle M15 |
|---|---|---|
| 32 Ω | Published | Unverified |
| 150 Ω | Derived | Unverified |
| 300 Ω | Derived | Unverified |
| 600 Ω | Derived | Unverified |
Questyle M15 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 Cayin RU7 column does stand on a power rating its maker printed, at one of these four loads at least. Reading the two straight across sets one kind of claim against another, and nobody has written down the conditions of both side by side.
2 headphones reach 100 dB on the Questyle M15 but not on the Cayin RU7 — Symphonium Crimson, Symphonium Helios.
563 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: Questyle M15 has +13 dB of headroom there against +8 dB. 5 dB is about 3.3 times the power, which matters for peaks rather than for how loud the music sits.
The hiss columns are empty for Cayin RU7 and Questyle M15 because neither maker publishes no output noise figure.
| Headphone | Impedance | Needs | Cayin RU7 headroom | Questyle M15 headroom | Cayin RU7 hiss | Questyle M15 hiss |
|---|---|---|---|---|---|---|
| Shure SE846 | 9 Ω | 0.04 mW | +31 dB | +33 dB | not published | not published |
| Campfire Audio Andromeda 2020 | 9 Ω | 4.0 mW | +11 dB | +13 dB | not published | not published |
| Audeze LCD-2 Classic | 70 Ω | 0.79 mW | +20 dB | +23 dB | not published | not published |
| Beyerdynamic DT 1990 Pro (250Ω) | 250 Ω | 0.63 mW | +15 dB | +20 dB | not published | not published |
| HiFiMAN HE400se | 25 Ω | 7.9 mW | +12 dB | +15 dB | not published | not published |
| Audio-Technica ATH-R70X | 470 Ω | 1.6 mW | +8 dB | +13 dB | not published | not published |
| Sennheiser HD 600 | 300 Ω | 6.6 mW | +4 dB | +9 dB | not published | not published |
| HiFiMAN Susvara | 60 Ω | 50 mW | +2 dB | +6 dB | not published | not published |
The Cayin RU7 costs $19 more than the Questyle M15, and on these three axes the money buys nothing: among the 567 models here there is not one that the Cayin RU7 handles and the Questyle M15 does not — not on power, not on the one-eighth rule, not on hiss. The traffic runs the other way: the cheaper Questyle M15 drives 2 models to 100 dB that the Cayin RU7 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 Questyle M15 has published no electrical specification at all and everything above comes from independent measurement: Cayin RU7 and Questyle M15.