Short answer: whichever flaw you can live with, because both measurements are rough, though neither ranks well in its class. The Sehlin Helium is a palm-sized kit whose CEA2034 spinorama shows an uneven response the designer’s own re-measurements confirmed. The Definitive Technology ProMonitor 1000 controls bass distortion and can be pushed hard for its size, but loses bass energy between 40-120Hz and resonates around 14kHz. Both are rated below 3.0/5 among the 92 bookshelf speakers, and the Ascend Acoustics HTM-200SE2, ranked #7 at $368, is a better place to start if sound quality matters.
Buy the Sehlin Helium if the price matters more than sound quality. Buy the Definitive Technology ProMonitor 1000 if sound quality matters more than the price.
Want more options? Browse the full Passive Home rankings.
Specifications Side By Side
| Sehlin Helium | Definitive Technology ProMonitor 1000 | |
|---|---|---|
| Price | $140 per pair | $498 per pair |
| Ranked | #84 of 92 in Medium Size Bookshelf Speakers | #83 of 92 in Medium Size Bookshelf Speakers |
| Bass Extension (-3dB) | approx. 170Hz (measured, Audio Science Review) - low confidence - this tiny single-driver design has a highly irregular response with a deep notch around 30Hz, not a simple monotonic rolloff | 95Hz (measured, Audio Science Review) |
| Bass Extension (-10dB) | approx. 70Hz (measured, Audio Science Review) - low confidence - this tiny single-driver design has a highly irregular response with a deep notch around 30Hz, not a simple monotonic rolloff | 80Hz (measured, Audio Science Review) |
| Drivers | Dayton Audio ND91-4 3.5” full-range driver (single-driver design) | 1” aluminum dome tweeter, 5.25” BDSS mid/woofer, 5.25” planar bass radiator |
| Dimensions | 6.5H x 4.5W x 5D in (0.04 cu. ft. enclosure) | 10.88H x 6.25W x 6.5D in |
| Woofer Cabinet | sealed | passive radiator |
| Sensitivity | approx. 82dB (2.83V/1m, measured, Audio Science Review - low confidence, irregular response with no clear flat region) | 86dB (2.83V/1m, measured, Audio Science Review - reviewer noted this vs. a manufacturer spec of 91dB) |
| Impedance | 4 ohms (Dayton Audio ND91-4 driver) | 8 ohms nominal |
| Spinorama Preference Score | 2.58 (spinorama.org) | 3.2 (spinorama.org) |
| Spinorama Preference Score with Perfect Sub | 5.46 (spinorama.org) | 6.58 (spinorama.org) |
| DIY Kit | Yes | No |
| Weight | - | 7.3 lbs (3.29 kg) each |
| Frequency Response | - | 47Hz - 30kHz (Manufacturer specified) |
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Prices on this page were last checked October 3, 2026 at 10:26 PM UTC. Product prices and availability are accurate as of that date/time and are subject to change; the price and availability shown on Amazon at the time of purchase will apply to the purchase of this product.
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The Two Speakers In Full
Sehlin Helium: $140 per pair eBay
Pros: An entire pair of drivers and crossover parts costs about $140, with a Parts Express flat-pack enclosure keeping the build simple: genuinely small enough to fit in the palm of a hand. Measured bass extension of 170Hz (-3dB), with a measured sensitivity of 82dB.
Cons: CEA2034 spinorama shows an uneven response that the designer’s own re-measurements confirmed rather than explained away as error. The reviewer described the sound as muffled, boomy, and gritty even up close, concluding “this is a failing attempt” relative to larger designs. Maximum output is limited to about 80dB SPL, well below normal listening levels for most rooms.
Full Sehlin Helium review and specifications →
| Bass Extension (-10dB) | approx. 70Hz (measured, Audio Science Review) - low confidence - this tiny single-driver design has a highly irregular response with a deep notch around 30Hz, not a simple monotonic rolloff |
|---|---|
| Bass Extension (-3dB) | approx. 170Hz (measured, Audio Science Review) - low confidence - this tiny single-driver design has a highly irregular response with a deep notch around 30Hz, not a simple monotonic rolloff |
| DIY Kit | Yes |
| Dimensions | 6.5H x 4.5W x 5D in (0.04 cu. ft. enclosure) |
| Drivers | Dayton Audio ND91-4 3.5” full-range driver (single-driver design) |
| Impedance | 4 ohms (Dayton Audio ND91-4 driver) |
| Sensitivity | approx. 82dB (2.83V/1m, measured, Audio Science Review - low confidence, irregular response with no clear flat region) |
| Spinorama Preference Score | 2.58 (spinorama.org) |
| Spinorama Preference Score with Perfect Sub | 5.46 (spinorama.org) |
| Woofer Cabinet | sealed |
- Links
- Reviews, Comparisons and Sound Samples
- Head-to-Head Comparisons
Definitive Technology ProMonitor 1000: $498 per pair Amazon eBay
Pros: Bass distortion is well controlled per the measurements, and the slow bass rolloff means it can be pushed fairly hard for its size. Ships with a footer that keeps the small cabinet planted despite its light weight. Measured bass extension of 95Hz (-3dB), with a measured sensitivity of 86dB.
Cons: CEA2034 spinorama shows severely reduced bass energy between 40-120Hz, a directivity mismatch between the tweeter and woofer, and a severe resonance/interference issue around 14kHz. Distortion at 96dB SPL was so bad the reviewer limited testing to 86dB. Direct comparison to a $118/pair Pioneer SP-BS22-LR left the reviewer calling the $249/pair ProMonitor 1000 “a bad experiment that should be forgotten” and concluding “there is nothing Pro about it.”
Full Definitive Technology ProMonitor 1000 review and specifications →
| Bass Extension (-10dB) | 80Hz (measured, Audio Science Review) |
|---|---|
| Bass Extension (-3dB) | 95Hz (measured, Audio Science Review) |
| DIY Kit | No |
| Dimensions | 10.88H x 6.25W x 6.5D in |
| Drivers | 1” aluminum dome tweeter, 5.25” BDSS mid/woofer, 5.25” planar bass radiator |
| Frequency Response | 47Hz - 30kHz (Manufacturer specified) |
| Impedance | 8 ohms nominal |
| Sensitivity | 86dB (2.83V/1m, measured, Audio Science Review - reviewer noted this vs. a manufacturer spec of 91dB) |
| Spinorama Preference Score | 3.2 (spinorama.org) |
| Spinorama Preference Score with Perfect Sub | 6.58 (spinorama.org) |
| Weight | 7.3 lbs (3.29 kg) each |
| Woofer Cabinet | passive radiator |
- Links
- Reviews, Comparisons and Sound Samples
- Head-to-Head Comparisons
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