Just for fun (I've done nothing about it so far) here's a sample (recorded on a little voice recorder) of my tubby steel wound strings from Bb an octave and a bit below middle C followed by lovely ringing copper wound from F#. For 170 year olds the copper sounds pretty OK and they're less than 3 foot long! http://picosong.com/nsHG

Old strings but most probably an unglued bridge too, in my opinion.

The touching as said RXD is also an option

Perfect example on why pianos are repaired

I have some tubby tones due to old strings I can make you a record, this is something else than there

You did not locate the source of the problem yet? push on the bridge, push on the soundboard, while playing

It's no coincidence it's only the steel wound that are tubby, surely? The copper fine?

that may be the reason I hear them with a so different tone than bad copper wire (while you have also bad copper wire)

must be corroded, etc in that case, did you test by pressing on the bridge, tightening screwsn pushing on the bottom of board to see it it is glued, etc ?

Mama Mia!! and I thought I had problems with some wound strings on my 1957 53" upright.
Do tell what make and type of piano is it?
I would recon the experts are correct it IS more than strings which are the trouble.