Those XC28's are entry level forks so will have pretty unsophisticated damping and rebound control. I also think the stanchions will be steel hence the weight.
I had a pair of those Psylos a while back and if I remember correctly, there's not much to them. Not sure what that clunk means, but getting a professional service from the likes of TFT will probably cost more than the forks are worth. If I'm honest, I didn't think the forks were very good...one of my memories of them was a huge amount of flex.
2015 Stanton Ti Switchback
2006 Specialized Demo 8
2013 Specialized Allez Sport
1999 Santa Cruz Chameleon
1994 Cannnondale Beast of the East (rebuild still in progress)