So it's not clear how your system is wired up (signal and power cables) in order to offer any concrete suggestions re: the hum. Regarding the phase change, I am also not clear where you are attempting to change that, thus no idea what the source of the nasty thump you are describing. In all likelihood, you have a ground loop somewhere, and I have to assume you started having this after buying the Spectrals, or it's not clear if that's not even the case. Standard operating procedure: for starters, make sure you star-ground all equipment to one power distributor, using equal length (and optimally, same brand/model) power cords and even interconnects; also test for continuity between the ground plugs of each power cord, and somehow assert they make contact with every component's ground prong. Other standard questions: do you get the hum from all inputs and all sources? Or is this something that, say, only your phono would exhibit (in which case it may be an entirely different beast, depending on how well your phono is shielded, and what the ground connections in that apparatus look like). Replacing the speaker cables should not have an effect on hum, but replacing interconnects (and what are they now, MIT? your signature seems to conflict with what you are describing, in that it doesn't mention Transparent cables at all) may, if your cables have a wire disconnected somewhere or something, or if we are talking about phono only, perhaps the phono interconnects are not shielded properly - again, phono would be an entirely different beast.
To answer your questions in detail :
All interconnects are Spectral-Mit except to Satellite Box and to Oppo BluRay. ( considered unnecessary extra cost )
Present speaker cable is TA but I am waiting for new Spectral-Mit Speaker Cable. I will have new Spectral-Mit Speaker cables in about 6-8 weeks. My Signature is ahead of the facts and I apologise if that was confusing.
All is grounded to one power distributor and all grounding on the plugs is correct.
Power cables are not all the same length and they are not all the same brand. The PowerAmp is on MIT Z cord which it is supposed to enjoy.
The "hum" is constant on all sources and on no source and does not change with volume settings.
There is no wire disconnected somewhere.
The Hum is worse on the right hand speaker ( 50db @ 1 ft ) than the left speaker ( 36db @ 1ft ).
The "Phase" issue refers to the "phase" button on the DMC 30S and its remote. This switches "Absolute Polarity" to the speakers. ( inverts polarity ).
Using this "phase" button to switch polarity / phase produces the nasty crack through the speakers.
I do always find significant difference with correct or incorrect phase settings and do check all new recordings where I do not already know what setting that producer uses. In those cases, the only way to tell which phase is correct, is to toggle on the fly while music is playing - something which is currently impossible - one would simply not dare do it. The only day to change phase currently is to turn Power Amp off, change phase, turn Power Amp back on.
It is useful to understand that the new Spectral-Mit speaker cables will NOT change the Hum issue nor the Phase issue because now I don't need to wait for those cables to arrive to find out !
So, I now have 3 questions from all this :
1. It would be very useful to know if the "hum" problem indicates something wrong with the Pre Amp or the Power Amp because both are under warranty at the present time but not for much longer.
2. Likewise, it would be very useful to know if the "Phase" issue indicates that something is wrong with the Pre Amp or with the Power Amp.
3. I live in England where we are on 240 volts nominal. All the front end components including the Pre Amp are on constant 230 volts regenerated sine wave power. The Power Amp is on the 240 Volts nominal and is a 240 Volt version. One would not normally "lift the ground" in the UK 240 volt environment. In fact, I have only ever seen manufacturers warning of doing this and insisting on mains power grounding. Is it safe and in order to lift the ground on the DMA250 when it is running at 240 Volts ? If this cures both issues it would be great but of course not if it is dangerous and not recommended practice.
Many thanks indeed !