Down firing subs, particularly heavy ones do not benefit much from spikes. Spikes benefit speakers when there is latteral excursion (front firing). This is why spikes make such a big difference for standard speakers, they couple the speakers more rigidily to the floor and thus increase their effective mass for lateral movement. The speaker moves less and becomes more efficient at moving air.
As to corner placement, there is a huge debate on this one. It generally does not yield the flattest frequency response and depending on the cross-over point may be unacceptable. However, corner placement does re-inforce the bass as has been stated and thus the woofer does not have to work as hard. Less work generally means less distortion. So the placement becomes a bit of a give and take. We do have a tutorial on speaker placement on our site. If you go to the listening room and click on speakers there is a link to download a short white paper on speaker placement.
As to corner placement, there is a huge debate on this one. It generally does not yield the flattest frequency response and depending on the cross-over point may be unacceptable. However, corner placement does re-inforce the bass as has been stated and thus the woofer does not have to work as hard. Less work generally means less distortion. So the placement becomes a bit of a give and take. We do have a tutorial on speaker placement on our site. If you go to the listening room and click on speakers there is a link to download a short white paper on speaker placement.