drinke,
I wouldn't worry about the 20-39 tipping over. I've had my 25-31PCi on carpet and cement.....doesn't budge on either. All its weight is in the bottom....very low center of gravity. It would take some effort to tip it over.
As for sound quality,I've never listened to the SVS boxes, however they're supposed to have the same performance. My cylinder shakes things wonderfully!
Astheticly speaking, it depends on your goal. As stated above, if you're looking for something to "blend in", the "box as end-table" approach may be the way to go. However, if you're looking for visual impact as well, go with a cylinder. It DOES make a statement!
![Stick Out Tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)