Not to the front ported speakers but it could affect rear ported speakers..mind you I'm not sure that it would be detrimental to rear ported speakers as the bass lift created by soffit mount could balance out the loss.
Thanks John, I'm doing my garage studio finally and have a set of m-audio bx8s that I wanted to soffit mount. Would they be better on stands. I could soffit mount my behringer truths instead? then I could have both maybe? I'm doing digital 5.1 surround now too.