Actually, this is a great analogue question - Most RIAA equalizers are designed for roll off about the same place as a full range speaker. Here's the wierd part - records can and do go lower. I would be curious if anyone knows why this is even though the spec's for the RIAA say otherwise - maybe the gain/db matching is different? I think subs have more to do with your own ear and the music you prefer: undertones and overtones plus the way the music is engineered are very important. If you're listening to Living End or Charles Mingus you'e going to want one. If you like cello chamber music you are as well. However the only two good subs I've heard with crossover's that work and don't detract or overly color upper frequency output are the Paradigms and the REL's (this only holds true for the Paradigms if you have their speakers as well). Haven't heard Merlin's though. Otherwise if you are using papercones or something, other than Merlin's, I would just stay away, because generally it seems to me speakers in that $4000-$10000 range are better quality anyway. In the under three grand range I really haven't heard anything better than the Paradigms, a point of some contention, but I think it's held up by the fact that every salesperson I've seen posting anonymously here and on positive-feedback has a pair. The Vienna Acoustics, Sonus Farber's and Opera Piega's of the world have enough extension as it is. The Velodyne's to me just seemed to make an under-enthusiastic whomp-whomp sound - a real one note wonder. Of course if you're in a condo or apartment, why bother? A little tube integrated and a pair of bookshelves will fill up your room well enough. With analogue, it will be musical and satifying.
