Yeah, the Presto software download from Sonnet's site has 2 enablers for OS8 but I had seen a couple of other forum posts in other places where people mention a System 7 enabler, so I mailed Sonnet asking for it and they sent me it! It's also mentioned in the Presto Plus installation pdf that's on their site, so it definitely does exist!
But what use it is, I don't know. When the 7 Enabler is in my System folder, nothing happens. Junk it out and just have the Presto extension, it boots. But still doesn't work.
I actually have 2 versions of the Presto extension, v3.1 (1996-1998 Sonnet Technologies) (114Kb) and v3.2.1 (beta 1996-2000 Sonnet Technologies)(152Kb). The 3.2.1 is the one that Sonnet mailed me, 3.1 is the downloaded one from their site. 3.2.1 causes a "bus error" when loading the extension, whereas 3.1 just doesn't load/enable the card.
Can you check which extension you are using? If you mail me yours and it doesn't work then perhaps there is something wrong with the card.