Thank you, I understand this now. So IRSS is not "watertight", but it is better than the Microsoft driver.If IRSS didn't receive or understand the SIRC command (ie. if your amp happened to be located on the opposite side of the room from the PC's MCE-compatible receiver, or IRSS didn't support SIRC), you'd experience exactly the same symptoms as you have with the "MS driver".
This raises the question of whether MP should be using IRSS by default. You are better qualified to judge this than I am, but I think that my problem is sufficiently rare that it is probably not worth making a change. Also, this problem is now identified, and the solution known. So this can be explained to anyone else who encounters this problem in the future.
One-for-All is a very popular brand in Europe, and their remotes are generally cheaper than those from Logitech Harmony. So it is a shame that they suffer from this problem. But they are certainly not alone. The first universal remote that I used with WMC was an MX850 made by Universal Remote Control of America. The MX850 is an advanced remote control, but it too suffers from the same problem as the One-for-All.
(For completeness I will test the "General HID" support later today, and post the results.)
-- from CyberSimian in the UK