I'll give them a call tomorrow thanks, if I haven't been able to figure it out before then. I really don't want to send the amp back to Exposure if I can avoid it. I dont want to throw hundreds at it and frankly I worry that it might come back sounding different.

Ideally, I'd like to have the satisfaction of bringing it back to full working order myself. If not, I'll probably use the local company I mentioned. They build their own amps and service a lot of high end kit. I know first hand that they have succeeded with some esoteric amps that the authorised service depts have failed with. I feel like I will have more control over the work if it goes to them and I avoid any carriage risks too. 

Well, they do say a picture tells a thousand words! But from what you say it seems the supply is just a raw dc feed from the main supply. The pre "supply" separate board/regulator might be in the pre case or on the main board - or it may not have any.

My own thoughts are that this setup is almost entirely dual mono and the fault is the same on both channels, so I'm wondering if it's more likely to be one of the very limited shared components.

These are the transformer, the on/off switch and a small few components on the preamp boards. There are also a couple of what look to be bridge rectifiers on one side of the transformer. They are different sizes and shapes. I can photo them too if it helps.

This might take a bit of doing because I'm still afflicted by IoS 8 copy paste bugs but I will try and post some pics of the preamp boards

And the preamp output board

Here's a general one of the power amp. Almost entirely dual mono apart from the on off switch, transformer and two bridge rectifiers as far as I can see. The caps aren't fixed in place yet because I replace the originals with larger ones I had just to see if it made any difference. The power + and - each run directly to the smaller cap terminals on each side. One to the left, one to the right.

I'm so clueless I'm not sure how to find out. I can tell you that the power is carried straight from a - pin direct to one of the smaller smoothing caps. Also straight from the + pin to the opposite side's cap. I don't see any components anywhere. I think its a very simple arrangement. I have a Dual IV with 9a dedicated separate VI dual supply and a dual rail VII preamp. This more simple arrangement betters it sonically though, and I reckon its the finest sounding Exposure combo I've had.

I see what look like two separate rectifiers: one green and one black, both in my last pics. Have you any idea which one would be the most likely candidate to replace and also what I'd replace them with. One has a code on it but the green one is blank. It would be a fairly easy task to replace them as long as I knew what to replace them with. AFAIK they are cheap to buy and easy to do, so it couldn't hurt to try.

If that doesn't cure it, I'm really only left with the power switch and the transformer that are shared by both channels.

I've just noticed there is a PCB next to each PA heatsink, can you confirm what these contain ? Possibly protection relays ? I find it difficult to believe the preamps supply is unregulated but thats the difficulty dealing with such old & leftfield gear, I reckon the DIP14 IC's are regulators in the preamp.

If it were me I'd replace the rectifiers first, neither transformer or power switch can be at fault of that I'm certain. These will be fine: www.maplin.co.uk/p/kbpc2504-25a-bridge-rectifier-ar85g if the problem persists I wouldn't rule out a grounding issue, you really need to make up a new pre>power DIN cable to rule that out too.

Doesn't explain the whine being at the same level irrespective of volume setting.