According to Sheep's last post it clearly shows more output potential with the Velo. I say keep it start enjoying it and tweaking for best performance.
A 10" driver can easily play deeper than a 15" driver, if the 15" driver is in a higher tuned vented enclosure. The 15" will have more output over its operating bandwidth by a pretty large margin, but a 10" (vented) tuned lower could play deeper. However, it will have to struggle a bit at those low frequencies to produce higher spl's. If the 10" and the 15" were in equally tuned, vented enclosures, the 15" would appear to play deeper as it can play the frequencies below the tuning frequency at a higher spl until the rolloff or infrasonic filter of the system renders them inaudible.
Depending upon the tuning frequency of the Velo, it is entirely possible that the 10" SVS plays a bit louder at low frequencies near it's native tuning frequency (around 20hz). Once the subwoofer's are "in room" this difference will disappear some.