We are concerned in this article with controlled linear time-invariant multi input multi output systems in continuous-time. We study here the preservation under strict positive real substitutions (of zero relative degree) of coprime factorisations as well as stabilising properties in parametrised controllers. We also tackle here both the preservation of well-posedness (of the tracking feedback control scheme) and the preservation of H _∞-optimality properties in some classes of closed-loop systems affected by multiplicative unstructured uncertainty, including the so-called suboptimal H _∞-control problem by stable controllers (i.e. preservation of both strong stabilisation and H _∞-boundedness).