Roger,  This problem is very common on old upright pianos.  You have
either (1) a loose bass bridge, (2) or the bass strings have corrosion
and dirt on them, that keeps them from vibrating properly, or (3) the
bass strings will have to be replaced.

A loose bridge can be very hard to detect.  The bass bridge is held to
the sounding board with three screws and hot glue.  If the glue joint
goes bad, this will cause the problem even if the screws are tight.
Sometimes you can detect this simply by sitting on the floor in front
of the piano, playing the bass notes while pushing toward the
soundboard on the bridge with your foot.  If the tone improves, you
will have to take the bass strings out, remove the screws from the
back, and re-glue.

Sometimes you can improve the tone of old bass strings by Taking them
loose and looping the string over itself (see the Reblitz book) and
moving the loop back and forth to loosen the dirt.  After this is done,
clean them with steel wool to get out the rest of the dirt.  As you
return the strings to their hitch pins, twist the bass strings in the
direction of the windings.  About one turn on the double unisons and a
half turn on the single strings.  If this fails to improve the tone,
the strings will have to be replaced.

A good bass string will sound somewhat "metallic" when plucked.  You
can compare your "repaired' string to the one next to it in the scale;
there should be a marked difference.  Another remote possibility: mice
love to build nests in the ends of the keybeds and they can pull out
all the front key punchings.  This can cause the hammers to block
against the strings, especially in the bass.  Hope this helps.

Many thanks for all the advice about replacing my Keytops.

Andy Taylor
Tempola music rolls LTD