Hi,

curious about this one indeed... I have no real solution to offer but I wonder about a few things....
But beforehand, I assume you know your way arouds in electronic.
How did you measure the value of those caps? (Which instrument and were the caps still in circuit or removed or at least isolated from the circuit)
Very often these caps are somehow in series or parallel with a coil and some high value resitors.
Also you mention in your post that there is no large spikes on the house supply. How do you know for sure?
I am thinking here lightning strike on the power grid nearby your home, a neighbour with a 500Mwatts arc welder for hobby purpose!!

etc.. I found also that anything that use a compressor ( A/C unit, fridge, freezer etc...) produce quite a high level of spikes on the house power system.
A compressor is not like other motors, say like the dishwasher or the washing machine. A compressor at start-up is under a rather heavy load and appears like a jammed motor for the first few cycles...
Please understand that my questioning is really not to quibble about your knowledge or personal IQ

In my home, I have a completely separated sub panel for the studio.
I start from the main house panel with a 240Vac 60 amps fused cutoff switch, then go to a dedicated sub panel in the studio. This sub panel then holds 4X 15amps breaker that ends in 2 duplex outlet each.
The main computer and the audio rack are on the same UPS. I use a UPS not much for the 10minutes of power they can supply during a power failure, but they provide the best protection during brown-outs and also a very efficient line filtering.
The other lines are fed trough a 'corcom' EMI/EFI filter box and to a duplex wall outlet.
Since I have wired the studio this way.... never had ANY problems related to the AC power feed!!
Oh, I almost forgot, I have a rather thick ground wire ( I think AWG 6) welded to a copper bar in the studio and to the cold water copper pipe that feeds the house water. If that is not a good ground point....
Luc