Both are dongles, $269 against $250. This page puts them side by side on the only three things a maker can be held to — power, output impedance and noise — and then asks the question that actually matters: which headphones do they treat differently?
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 |
A figure marked not published is missing from the maker’s own material, which is not the same as being good or bad. Where one of these two publishes a number and the other does not, the comparison on that axis cannot be made at all — and saying so is more useful than guessing.
| 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.
What each of these labels means, and why a derived figure is still worth printing, is set out in how this data was built.
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. That is a power count and nothing more; whether either source suits a given headphone also depends on the two lines above.
The counts above say how many models the two treat differently. This says by how much. The same 8 headphones appear on every page of this kind, ordered from the easiest to drive to the hardest, so a row can be read across pages as well as across columns. 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. That is a gap in the specification, not a quiet amplifier, and it is left blank here rather than filled with a plausible number.
| 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.
Everything in this paragraph counts only figures the two makers published. An advantage on an axis one of them leaves blank cannot be counted, and is not counted — how often that happens is its own page.
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. Or put either against your own headphones in the source comparison tool.