Is anyone here still using AKG C451 mics, or remembers using them?

I've had my 4 for some time but am becoming more and more aware of a low
level hum on the output. The hum gets worse the more of them that are
plugged in to the mixer. If only one is faded up, the hum gets worse as
others are plugged in. At first I thought the phantom power supply was
giving trouble but I have now tried them on two different mixers and a,
stereo, standalone mic pre-amp (M-Audio) with similar results. I wondered
if one of them was drawing too much current and dragging the PS down but it
doesn't seem to matter which combination of mics I use. I think the hum is
100 hertz, which still suggests PS but on three differant i/ps?

Both mixers and the standalone are fine when used with dynamic mics.

The C451s are long in the tooth so are they prone to draw high current as
they age? Or am I barking up the wrong tree altogether?

Any suggestions, polite ones, would be appreciated.

Thanks.

Alan S.