I am curious how you are connecting the multiple inputs to the Xantech connecting block as I suspect that is the source of the problem.
A Xantech Connecting block normally expects to see the signal from a Xantech receiver output which is a lot like, but not exactly the same as an IR signal used to drive an emitter. The difference is in the driver circuitry of the receivers. The xantech receivers can be paralled into a buss like architecture and the paralled outputs (the buss) can be connected as the inputs to a connecting block giving multiple inputs. The receivers have special circuitry to allow this similar to tri-state logic or diode isolation. Treating the outputs of Philips extenders as if they were Xantech receivers will probably cause loading problems and may work depending on the final controlled equipments ability to deal with degraded signals.
Xantech makes a device called the serial combiner (793-10) which might solve this problem, but I am not sure. It provides diode isolation between multiple IR like inputs.
I can not say for sure but it would be an interesting experiment to hook one up and see what happens.
