Short answer: the Onyx Studio 9 for connectivity and balance, the Zealot ZE21 for value. The ZE21 is great value for bass performance, play time and max volume in the boombox class, though the ZE41 now beats it, and it is a little rolled off up top and not that balanced. The Onyx Studio 9 adds Auracast multi-speaker connection alongside classic stereo pairing, but has slightly recessed mids and rolled off upper treble.
Buy the Harman Kardon Onyx Studio 9. It is rated higher for sound quality than the Zealot ZE21 and costs less, so only choose the other one for a particular feature it does better.
Battery life favors the Zealot ZE21: rated for 40 hours against 8 hours for the Harman Kardon Onyx Studio 9.
If water resistance matters to you, the Zealot ZE21 at IPX6 (powerful water jets) holds up far better than the Harman Kardon Onyx Studio 9 at no water resistance.
Still deciding? The full Portable Bluetooth rankings cover every ranked option.
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Specifications Side By Side
| Harman Kardon Onyx Studio 9 | Zealot ZE21 | |
|---|---|---|
| Price | $184.99 | $187.15 |
| Ranked | #15 of 113 in Boombox Size Portable Bluetooth Speakers | #18 of 113 in Boombox Size Portable Bluetooth Speakers |
| Bass Extension (-10dB) | 41Hz | 36Hz |
| Maximum SPL | 101dB (measured by Alan Ross Reviews) | 116dB (Manufacturer specified) |
| IP Rating | None | IPX6 |
| Power Input | AC Power Cable (2-Prong) | USB-C |
| Stereo | Yes | Yes |
| Omnidirectional | No | No |
| TWS | Yes | Yes |
| Weight | 9.3 lbs (4.2 kg) | 20.4 lbs (9.27 kg) |
| App | Yes (Harman Kardon One app) | No |
| Bluetooth | Yes | Yes |
| Bluetooth Codecs | SBC | SBC |
| EQ | Yes (App EQ) | Yes (multiple modes) |
| Party Mode | Yes (Auracast) | Yes (PartyAdd up to 100 speakers) |
| USB Playback | No | Yes |
| Bluetooth Version | 5.3 | 5.3 |
| Battery Life | 8 hours (Manufacturer specified) | 40 hours (Manufacturer specified) |
| Amplifier Power | 50W RMS | 250W |
| Dimensions | 11.4” x 11.3” x 5.1” (289.2 x 287.5 x 130 mm) | 16.7” x 10.3” x 14.3” (424 x 262 x 363 mm) |
| Drivers | 120mm woofer + 3x 20mm tweeters | 130W subwoofer + 2x 50mm woofers + passive radiators |
| Digital Aux Input | No | No |
| Aux Input | No | Yes |
| Strap | No | - |
| MicroSD Playback | - | Yes |
| Power Bank | - | Yes (USB-A + USB-C) |
| Built-in Microphone | - | No |
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