"B103" typically means 10Kohm.
What leads you to believe they are 50Kohm?
I have a PS 28 III / AV18 Bose Lifestyle system that I'll be tearing into at some point.. When I was looking around for schematics for the Series III systems, I couldn't find them, but the Bose Series II schematics are readily available. Just look around. There's a database of service manuals somewhere that I saw them in. If I find the link to the site, I'll drop it here for you.