^ These MA40's are clinchers, not tubular rims. I have a set on my Trek 760 and IMO they are pretty good wheels.

Can't speak for what is awry with your rear hub.
I don't think the bike has a cassette - it's likely a FW.
There are plenty of 6-speed options for replacement if that is needed. If it is needed, I'd opt for a new one with hyperglide ramps and a new 6-spd chain to go with it. Niagara typically has the best deals on them.