First, it is absolutely brutal to declaw a cat (or a kitten for that
matter). Declawing is actually the amputation of the first segment of
the toe. It is not only painful (akin to having your fingernails
pulled out), but it ruins a cat's balance and coordination; weakens
the ligments in their legs (clawing is a form of exercise) and makes
the first response of defense not a swipe with the paw but a bite.
(Personally I'll take a small scratch any day of the week over a
bite!)
Second, you don't have to have them declawed if you provide them with
an alternative to your harp and your furniture. Buy them some rug
covered cat furniture (which can be found at any pet store) preferably
the kind with perches and hidey-holes and put it in front of a window.
Immediately they have a much more attractive alternative.
Praise them when they scratch on their furniture. A loud sound like
clapping your hands or a squirt from a water pistol will discourage
them from any experimentation else where. (Never hit them for bad
behavior, it doesn't work and olnly makes them aggressive.) If you are
unwilling to take a chance on your harp during the "learning phase"
cover it with a large plastic garbage or clothing bag when you are not
using it. Plastic always discourages them from clawing. The same holds
true with the arms and sides of chairs. You can take saran wrap and
tape those areas with masking tap. After a couple of weeks the cats
will ignore these areas and you can remove the plastic. There are also
sprays you can buy that will repel them, I personally don't care for
them.
Also trim their claws periodically. The vet can show you how. Trimmed
and filed claws do a lot less damage!
I have taken in about 30 cats and kittens over the years, from strays
to show cats, and I have never had a problem with clawing on the
furniture or my harp, following these methods. My current (3) cats
with claws stay indoors, on their own, all day long and they are
purrfectly behaved!!!