I am in the same boat with your friend. Stuck in a bedroom size 3.8 x 3.8m. Denon 1905avr, Wharfedale Diamond 7 channel surround with 12" REL sub. And currently just add CA 640A V2 into the family to drive the fronts. Ohya, don't forget the bed and wardrobe but omit the toilet, haha.

The result? Hard to get good bass as low frequency travels long distance. BUT, a friend of mine recommend me to lift the sub like 1 to 3 feet in order for the bass to travel longer. According to him this little trick works in SMALL room but don't practice it in BIG room. Well, I have not tried myself since finding a dedicated stand / stool for it seems troublesome.

Tell your friend, it is not how big a room we have, but instead what we do with the room

Because of room limitation, I would say around 4m in length is overkilled for a huge sub, 6m in length would be ideal. But I have no idea on SVS sub. Because of the short distance, low frequency only able to resort to bounce front and back in the room which resulted in bad bass. Like all of of knew low frequency is best when it is felt but not heard.

One could elevate the sub by using a stool, rack or stand provided it is rigid. Try to experiment in the range of 1 feet to 3 feet. That will give more room for the low frequency to travel. And one important thing, do carpet the listening area, it helps.

Tun Mega, my sub is not down firing. It is front firing, sealed type. I put it facing the wall cause it does not comes with a grill and afraid might just terkick the driver, hehe.

Am no expert, just to share opinion. Worth experimenting for those whose has space limitation.

AGREE with horny!!! This is what I do. Blast kaw kaw when u are alone at home!