A condenser head will not fix underexposures. The sort
of partially diffusing condenser found on these enlargers
produces about one paper grade more contast than one would
get on a diffusion enlarger. I think I get less than that on
my Omega D2v.
   Increasing the contrast will separte details in the
shadows somewhat because they are recorded at lower contrast
than more fully exposed parts of the image, but the entire
image is being printed so that part which got full exposure
will be overly contrasty. At some point of underexposure
there will simply not be anything in the shadows to recover
although its surprizing how thin a negative can be and still
print.
  Increasing contrast in printing will also exagerate any
blemishes or dirt on the negative. When high contrast paper
is used it will also exagerate any uneveness in the light
source. All things to watch out for.
  One method of getting reasonably good prints is to make
masks and print different parts of the image with different
variable contrast filters (on VC paper of course). This is a
lot of work but can give you results not possible in any
other way.
  If you already have a good tungsten diffusion head or a
color head you can probably get enough contrast by using the
highest contrast filters. For a color head, the use of an
actual VC filter will often give you a higher contrast than
the built in filters.
  Cold light heads will generally compress the contrast
range at both ends although its worth a try if you already
have one.
  I have managed to get prints of what I call salvage
quality from negatives which looked nearly clear with a
little care.