After you've visited Trash Palace, Mickes and Stockholms Serie & Skivhandel, these 2 newish shops also deserves a visit:
at Hornsgatan 153 (T Hornstull) map
Combined retro/trinkets- and vinyl shop with a good Metal selection.
An Ideal For Living
at Södermannagatan 19 (T Skanstull) map
Another retro/trinkets/records combo. The basement is where you'll find one of Stockholm's best selection of Metal vinyl. I usually find some cool stuff here every visit.
---and an old store that has gotten a lot better on the Metal vinyl front in recent years:
at St Eriksgatan 70 (T St Eriksplan) map