I am not familiar with the cable pro, but I don't think it is helping or hurting the issue here as I am reasonably sure the issue you are dealing with is not power line related as it sounds like you are syaing that indpendant of whatver power cord or power line conditioning you have used, that the noise is still there in the same manner.
Lets move onto your amps... The MKVs are the quietest OTL Joule has made. Having said that, that does not mean these amps are dead quiet. Thier design (floating power supplies and the circletron design topology) mean that if things are not perfectly in balance, some noise is going to be generated and this is uusally the case. Also there can also be some variability between pairs... some being a little more quieter or noiser than others. I have knwon Jud to do some tweaking (probably fiddiling with grounds inside the amps) to some pairs of OTL to get them a quieter. I have heard a few pairs of MKV version Joule VZN-100 OTLs on VSMs and the noise level is pretty low on these amps... requiring you to be standing next to the speaker to be able to hear the noise - these amps though are never dead quiet though (no amp is actually and the all have varying amounts and types of noise) and the more effiecient the speaker, the greater the problem. Having said that, you may be able to get things a little more quieter by getting the balance between the two halfs of each amp better matched. I would like you to check the DC offset - that extra button near ther bias buttons (no music , inputs muted) of each amp and let me know what each amp is reading. Depending on what you are reading there I amy have you tweak bias settings to get things dialied in a little better which may help reduce the noise a bit.
I can imagine in a setting where the noise floor in the listening room is low enough, that even a fairly quiet pair of VZN-100s when hooked up to a efficent speaker like the VSMs will have a noise level that is audible from several feet away. What hurts here is that the noise tends to be buzzy in nature which is more apparent to the ear in nature than a hum.
Now, there are seom possible issues we have not considered here....
1) RFI.. probabaly not a issue here..
2) Power line issues (which convential power filter type products you have used cannot possibly correct).
3) There is something amiss with the common variac power supply unit that is causing both amps to be more noiser than they should be... never seen this before and I doubt this is the case.
Happy Listening,
Rich Brkich
Retailer & Audio Asylum Industry Liaison