Well, you can start with the driver surface areas. An HGS18 uses an 18" driver, so you've got 81*pi. SVS uses 12" drivers, so you've got 36*pi. However, I would guess the SVS drivers are newer and a little more capable of further excursion. More likely this is the case with the Ultra drivers. So two 12" SVS drivers.
The HGS18 is also spec'd at 1250W RMS, 15Hz as its low point. I'd say you'd be looking for a SVS PB12-Ultra/2 which is 1000W RMS. About the same price new as an HGS18 is used, based on a current Audiogon posting. The PB12-Ultra/2 has a 16Hz tune mode, which matches the HGS18. But it looks much nicer and is also much heavier, which is partly because there are two drivers, I would imagine, and also because it is bigger and has more internal bracing.
However, the HGS18 is sealed, while the PB12-Ultra/2 is ported. Which may make a difference for some people.
My personal choice was a pair of 16-46PC+ subs, because I wanted the 12Hz tune mode but one wouldn't have provided enough SPL for me. Now I'm just looking for something that can cover that last 1-10Hz range.
