The passive amp is actually favorable for Hsu because they do not need to modify their cylindrical outer wall in order to provide built-in amp. If you see the Hsu sub in person, you will notice very little cabinet resonation and flex, even in comparison to other subs.
The top mounted woofer on the Hsu does add concentration of weight towards top. There must be a good design reason for this orientation though, because Hsu has built down-firing cylinders in the past and claims that this version is the best. One interesting effect of this design is that no grille is required at the top of the subwoofer. A grille can impart a slight coloration to the sound. On any cylinder sub with ports at the top, a grille is a necessity so that loose objects do not fall into the ports.
The comparison of rated amplifier wattage is useful only if everything else in the design is the same. This is never really true. The Hsu uses an extremely long port, a very non-resonant structure, and overdamped alignment to help achieve prodigious output. A single TN1220HO used with 250 watt can shake almost everything in the room at frequencies as low as 15Hz, very cleanly too.
Variable tune is good. The Hsu is tuned so low that it certainly does not need any more extension, and it has a very good combination of clean output and extension. The next generation of TN-X subwoofers will have ability to variable tune and ability to be kept so that concentation of weight is centered more towards the bottom of the sub, but release date is very unclear.
Parametric eq is sometimes good. New user need to be careful when using a built-in equalizer. Using equalization to smooth and flatten frequency response at the primary listening position can sometimes be detrimental to the response signatures at other listening positions, so caution must be exercised. The next generation of Hsu subwoofers may see this as an added feature, but again release data is not certain.
Some of Hsu's trump cards when used with custom built 500 watt amplifier include:
> Audiophile-grade crossover (class A)
> Powerful and efficient amplification (class H power 500wattsRMS x 1 into one subwoofer, 400wattsRMS x2 into two subwoofers, 800 watts total effective) that can power up to two TN sub
> Feature filled amplification (soft-clipping, audiophile-grade high pass and low pass network, filtering of subsonic frequency)
> Easier ability to place sub on the floor on it's side and out of view
> Very little cabinet resonance and very little coloration with no built-in amp and no grille.
> Very easy to move around because of light weight, and very easy to place in the room because of little use of floorspace
> Extremely tight tolerances and very low and extended bass response as tested by industry professionals
> Extremely clean output with no hint of noise from ports when driven to prodigious bass output levels of greater than 110db's\\20Hz
> Ability to replicate music with accurate tone and definition and with massive headroom and output capabilities translates very well to the Home Theater environment where synthesized explosions are mixed in with more delicate and musical soundtrack on DVD.
I must add that ADIRE has a very well-tested cylinder subwoofer called SADHARA with wood finish, advertised with highest excursion driver current available, probably worth checking out.