Yamaha Upright 'phase' problem in Bass

[Woodrow, John (Parramatta)]

List,
I am experiencing a problem sorting out what sounds like a bicord 'out
of phase' problem on an older U3 series Yamaha upright.  It is the first
bicord on the tenor end of the bass bridge (Bb).  The soundboard has
minimal crown and there is not a lot of downbearing (don't know how much
crown they had to start with).

The bicord is a curse to tune.  Never seems to be obviously right.  The
rest of the bass is a joy to tune.  It just constantly has that 'out of
phase' sound.  Tuning only minimizes it.

Tried so far:
Shaping the angled hammer for better alignment to strings - definite
improvement
Seating strings at Bass bridge - think that improved things a bit.
Replacing both strings - no noticeable improvement in the 'out of phase'
sound but definitely easier to 'tune'.

No obvious bridge problems.  Have run out of ideas so conducted a few
field tests on older U3's.   Every U3 of similar age that I have tested
(15 years old) seems to exhibit this out of phase sound on this note to
some degree, though no where near as bad as the piano in question.

So my question to the list is:
Has anybody else run into this problem?  Am I overlooking something
obvious? Is it a 'feature' of this make/model that is exaggerated by a
loss of crown?

Appreciate your help.

Regards,
John Woodrow
Sydney, Australia