It occurs to me that the problem with loud trombones may be nothing
to do with the pipes themselves.  Trombones are on their own register
and are brought on by the arranger at specific times in order to
accentuate a bass line in the music, or to add gravitas to a passage.

The problem here may be that the organ has been altered and the
separate trombone register has been removed, or maybe the music being
played has not been arranged by a sympathetic traditional arranger but
that the organ has been converted to MIDI and is playing stock
arrangements that have not been correctly adjusted for that specific
organ.

Before trying to quiet the trombones check that the registers are
correctly connected to the relevant sections of the organ and that the
organ is playing from cardboard music arranged by only the best
arrangers.

Nicholas Simons, GB