Darkangel:
1991 is more shoegazy and borrows quite obviously from MBV's guitar feedback and noise, its not totally shoegaze nor grunge but somewhere in between. It is a collection of their earlier unreleased works.
No Return in The End is their last and latest album. I don't think it sounds like Nirvana but it definitely has the grunge sound. Rob Montejo definitely is a better singer and rarely growls or slurs, I think that is one of the things that makes SO stand out.
Yes, their album are extremely cheap used or new, I found No Return in the End new at around $4 at the local record shop, 1991 can be bought new at $10 at elephants records' website. I guess that is because they aren't wildly popular. They kind of loss some fans when they leaned towards a more grunge sound in their later album. I found No Return in the End and 1991 to be of a very different sound, so hopefully you will enjoy them. I like Rob Montejo's new band (duo) The SkyDrops as well. Stripped down guitar and drums sound, no other instruments, but its has very textured sound.