Hsu vtf3 mk2 20 Hz
By plugging one of the ports one achieves around 4 Hz more extension. The shape of the frequency response is still quite good, although the same upper end roll off can be seen. Therefore I wouldn't use it with much higher crossover than 80 Hz. The built in crossover doesn't go higher than ~70 Hz. The lowest setting results in very low ~25 Hz crossover. The maximum mid bass output is lowered by at least 2-3 dB compared to maximum output mode. The deep bass compression goes even worse because now there is only a single 3” port left. THD profile is similar to the 25 Hz tune with high deep bass distortion at high output levels, and upper bass distortion anomalies/resonances. Group delay stay low down to 28 Hz, but grows rapidly below. Spectral decay shows a lot of ringing near the tuning frequency. Spectral contamination is similar to the 25 Hz tune, though with slightly higher overall distortion. The overall build quality is average.
+ Really good extension and relatively flat frequency response
+ High mid/upper bass output capability
+ Overall price/performance ratio
- Somewhat high deep bass distortion at high output levels
- Distortion anomalies/resonances in upper bass range
- Ringing around the tuning frequency
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