That's definitely not a good idea. Not all the sound a sub makes comes from the woofer. All SVS subwoofers are ported, and a lot of the bass sound comes from the port(s). You would seriously mess with the sound of your SVS by mounting it in your attic.

If the size of the SVS is really that large a WAF issue, I'd consider getting a smaller sub before building an attic enclosure for it.