There are several possibilities each of which can be the cause of your
problem.  Also all of them could be the problem.

The shape of the bass hammers will dull the clarity of the bass line
on any piano.  They should be sand-filed so that there is a pear shaped
hammer with a slight point on the crown.  This will cure the problem
only if the piano is otherwise in tip top shape.  This is probably not
the case.

The strings are probably going dead, that is the triangular spirals
inside the string between windings and core are filled with black
patina and the string is no longer flexible.  This can be somewhat
cured by taking them loose on the hitch pin and bending them back and
forth all along the length of the winding.  Some techs tie a loop in the
string and pull it down to about 4" in diameter and pull it up and down
the length of the string.  When you reattach the end of the string
twist it 3 times in the same direction as the winding so as to tighten
it.  This may cure the problem.

If this does not cure the worst offending string then the problem
is the bridge is unglued. This is the case when the volume fades out
as you play chromatically from middle C down.  Sometimes there is a
drastic drop in volume as you reach the tenor/bass break.  Sometimes it
is just gradual.  The bass on any upright should boom like on a grand.

Remove the bottom end of all the bass strings and notice how the bridge
falls off the apron.  Remove the apron from the soundboard by taking
out the screws in the ribs or buttons on the back of the soundboard.
Wash all the old hot glue off the bridge and apron with hot water or
wet rags.  If there are cracks around the bridge pins then have a new
bridge made by John Trefz.

Glue the old or new bridge back to the apron and use 3-4 wood screws
from the back.  Glue the apron back to the soundboard and screw down
better than before using longer screws or an extra one or two.  Use hot
glue.  When you replace the bass strings give them the 3 twists to
tighten the windings.  If you go the wrong way then they will sound
worse than they were or they will buzz.

It sounds like your apron may have cracked and has come loose from the
bridge at one end.  No fear, if it is loose at one end the other end
should be popped loose and glued anyway.

Another problem, which is less likely, is that the soundboard is in
such bad shape that it doesn't even come close to having a crown.  I
had one lately that had all the ribs loose and the board was wavy like
a washboard.  It had no tone.  You could play hard and barely hear the
piano.  This of course needed complete re-stringing and we glued all the
ribs back and re-crowned the board.  It is now as good or better than it
ever was.

D. L. Bullock