Best subwoofer?

The REL Studio comes in three generations I, II and III (Linkwitz Riley, ARM and Super ARM loading). Has anyone managed to compare them, ie, is there a significant improvement to consider getting the later generation model? I'm considering to use one with the Apogee Diva's, where both speed and musicality are important.

Perhaps, you should have stated some constraints with respect to your question, such as size weight and cost. This is from the perspective that the best subs out there are very large (as a minimum, think 3' by 4' by 5'), generally well braced and massive (300-500# per side) and very expensive ($10-20k per sub). That is not to say that there are not good choices at other price points and with smaller size. Of the subs mentioned, I would recommend either the larger Audio Physics subs or the Martin Logan Descents. I have had very positive experiences with both, albeit in other audiophiles systems. I have also heard good things about the digital Velodyne self equalizing units but have no personal experience.

Any REL.

I have owned dual decents, many velodynes, HSU ,JM labs many many others and by far the best bass you can get is not with the compramised sub woofer designs but from a true bass system. Subwoofers are flawed in design from the start, you need massive power, large excursion drivers, subs need room and boundery reinforcement to produce deep bass. A bass system uses low excursion drivers can run on low power isnt effect much by the room. These designs can be horn loaded, BR, TL or even OB its only weakness is size and costs. But with what decents cost or REL. You could get real lifelike bass large excursion drivers cant reveil these small details like low excursion can, the details are lost to the massive forward backward movements these sub drivers produce.Why so many folks with great loudspeakers find subs to be very hard to integrate.Toss in room nulls and peaks that one must work with when subs are used its amazing to me that anyone can enjoy such sound. But as always YMMV

if you need a sub for a stereo system, the only one I have listened to which is fast enough to integrate with a <=9" woofer is the Wilson Benesch Torus