  Two-qubit X-matrices have been the subject of considerable recent attention,
as they lend themselves more readily to analytical investigations than
two-qubit density matrices of arbitrary nature. Here, we maximally exploit this
relative ease of analysis to formally derive an exhaustive collection of
results pertaining to the separability probabilities of generalized two-qubit
X-matrices endowed with Hilbert-Schmidt and, more broadly, induced measures.
Further, the analytical results obtained exhibit interesting parallels to
corresponding earlier (but, contrastingly, not yet fully rigorous) results for
general 2-qubit states--deduced on the basis of determinantal moment formulas.
Geometric interpretations can be given to arbitrary positive values of the
random-matrix Dyson-index-like parameter $\alpha$ employed.
