Tathagatagharba and
Madhyamika are (IMHO) not incompatibe , rather they seem to be complimentary.
Madhyamika, following Nagarajuna's lead, seeks to undermine the habit of the unenlightened mind to reify what it thinks it perceives as being what really is. Thus, it employs a radical apophatic method to clear the mind of mental detritus. It does not (again IMHO) reify the concept of emptiness, it rather tears down all the veils (filters and projections based on hopes, fears, attraction/aversion, etc) that we have created.
Tathagatagharba, on the other hand, proceeds from the clarified unenlightened mind acquired through
Madhyamika praxis to looking at reality from the point of view of the enlightened mind and says that with a mind unburdened by conceptual filters and projections we can see reality-as-it-is. Reality-as-it-is (suchness/
tathata) being that to which
Shakyamuni was awakened.
As
Madhyamika did not reify emptiness (which would lead to nihilism), so too
Tathagatagharba does not reify
tathata (though often accused of doing so) but rather breaks the distinction between subject and object, samsara and nirvana, and merely uses the term
tathata to point to what is left when the mind is cleared of all entanglements. That is,
tathata really exists because it is not an object relative to a subject able to distort it. Remember that
Madhyamika cleared the unenlightened mind of the husks of reifications within the subject-object manifold, so from there
Tathagatagharba eliminates dichotomous perception so that what is left is what is real but it is not reified as object for there are no psycho-conceptual filters or projections causing distortions - there is only the immediate presence of reality.
However, and this is the tricky part, one has to make sure that the noumenal unity of things does not blind us to the phenomenal multiplicity, even as (using apophatic
Madhyamika praxis) we had to avoid letting the mulitplicity of phenomena veil us from the unity at the level of noumenon. When both of these views have been integrated one may be called a Bodhisattva. When they have been fully integrated and made into a permanent and constant lived reality, then one can be called a Buddha.
So, taken out of some of the jargon: when one realizes that what we perceive is not precisely that which is but an 'image' we created based on the results of both perception and conception (and all our psychological baggage), then we will see that the objects of perception and conception are empty of independent substantial existence. Following upon that we might realize that the exact same thing is true of the self (or perceiving subject). When we have an authentic experience (not just conceptual understanding) of this, then we will see (as does Avalokiteshvara in the Heart sutra) that "whatever is form, is emptiness; and whatever is emptiness is form." From there we realize that samsara and Nirvana are neither two nor one and Buddha Nature (
Dharmakaya,
Tathagatagharba, etc) and unenlightened human consciousness are also not completely two nor indistinguishably one.
Having said this, there are many individuals who would disagree with this assessment and would rather posit a doctrine of negation in Madhyamika and a doctrine of affirmation in Tathagatagharba and say that the two are diametrically opposed. But I do not see Mahayana Buddhism as being based upon doctrines, but upon upaya and so my reading often differs from the majority reading. Therefore, you should probably take anything I say with a grain of salt

and read up on all sides of the issue and see what you make of it. But, be forewarned, such investigations will never yield anything worthwhile if alienated from the praxis, which -
Jodo Shinshu- is said to be 'given' by Amida.
If this does not help, may it at least do no harm.
Namo Amida Butsu_/\_
P.S. You may find that the text "Buddha Nature" by Sallie B. King to be useful in wrestling with these issues -
http://astore.amazon.com/bodhi.mine-20/detail/0791404285