In my opinion "dull" or "tubby" bass notes in old upright pianos is
mostly caused by a build-up of dirt and corrosion between the windings
and the core wire, or by a loss of down-bearing (caused by many
things, mostly loss of crown in the soundboard -- not an "easy fix").
I have had sometimes very good luck by loosening the strings (one by
one, so as to not upset the tension on the piano) until the bottom loop
can be lifted off its pin, then by taking the wire up above the keybed
(action needs to be removed for this process), coiling it onto itself
and using something like a wood dowel, run the coil up and down the
length of the wire; this over-flexes the wire and loosens the gunk.
If you're outside, or in a shop, blow compressed air over the whole
string; if it's in the house, use a vacuum cleaner to keep the mess
down. Then put the string back on it's pin, and turn it one turn in
the direction of the windings (just in case they have loosened up some).
Tighten the string, see if it sounds better; if it does, then do the
next string and so on. This will take a few hours, but I have found
sometimes a great improvement. This same process does work on grands,
except you don't have to pull the action, but you do have to work
around the dampers.
If you don't want to go that deep, take the action out, and violently
pluck the base strings to get them to really move around; you'll be
surprised at the dirt that flies out, have a vacuum cleaner hose nearby
to suck it up so it doesn't ALL settle back into the piano This will be
very noisy!! Sometimes this does a significant improvement too.
Having said all this, I will gladly admit that a new set of bass
strings does wonders, but if the downbearing is already lost, you'll
have to do more work than just change out the strings-which often also
requires replacing tuning pins & sometimes the pinblock.