Pianotech

I am new to the PTG (1 year) and this is my first post.

Client has a 1983 Steinway model 1098 45" Studio Upright (with Sostenuto).

First question:  The client commented about an "echo-y" sound when striking a key in the mid-tenor.  I found that there is reverb/ringing at the frequency of the note struck and released.  I further found that this reverb can be greatly reduced or eliminated if I simply put my hand on the base strings.  It seems that each single and bi-chord note is activated if the tenor note I strike is one of it's partials.  Makes sense - except the dampers appear to be properly set on all the bass strings.  I can press each base string and see the damper following it.  How would you address this?  Is it a matter of regulating the bass dampers so that they are pressing harder on the strings?  Should I 'rough up' the dampers a bit so they make more contact with the strings?  Or is there something else going on?

Second question (unrelated to the first I assume) is less important than the first since I am the only one who notices it:  The sostenuto feature works well in all ways but one - when you release the sostenuto pedal, even slowly, all the notes held are released somewhat far from the strings, which make an audible 'thump' when they snap back to the strings.  I have not yet taken a look at the sostenuto feature in the action to see if I can figure out how to regulate it.  Does anyone have any suggestions?   I Plan to return later this week.

Thanks!