Are there any ill effects from using the attenuators?A lot of people are happy with them, but there have been some reports of adverse effects on dynamics, etc. I suspect that they would do fine when driven from a component having very low output impedance, such as your Ayre preamp, and assuming that the input impedance of the amp does not vary significantly as a function of frequency (which I would expect to be the case).
I wonder if any Mcintosh power amps would work well with the Ayre?The key thing is that you want an amp having significantly lower gain than your present amp. More often than not that will correlate with having a lower maximum power rating than your present amp. The MC501 is an exception, though, having greater power capability but significantly lower gain. See JA's measurements.
The hum you are now hearing with rca's is most likely due to a ground loop problem, assuming that the interconnects are not running close to and in parallel with any power cords. I suspect that if you are hearing just hiss and not hum with the xlr's that the hiss is not being caused by ground loop issues, but you can verify that by using cheater plugs as DTC suggested. As I had indicated, xlr interfaces are inherently less susceptible to ground loop issues than rca interfaces, at least if they are well implemented.
Regards,
-- Al