[gramofiend]

If you are uncertain or unsure as to how much work you wish to lavish on a piece of equipment such as this you have also a choice to send it back to the maker. In fact,as far as Quad is concerned, I know that as far as the 405 power boards were concerned, the maintenance unit in the region of the BBC, where I worked, stocked six boards and did not bother to repair at component level! Returning a faulty board in a well padded bag or protected box to Huntingdon, Where the service department is, being well worth it. By the way you very rarely get your own board back but a guaranteed one in spec and fully working. The people working in that service department were often trained there and may be almost as old and experienced as some on this forum. It must be one of the few originally british companies, whose new Chinese owner decided to keep a service department in britain and to maintain equipment going back to valve days.

From my reading of what you say in post 1. a QUAD 405-2 power amplifier should produce no hiss or any noise even if placing your ear next to the speaker! You say you get hiss when the pre-amp is increased which indicates a volume control which a 405-2 does not possess so which preamplifier is in front of the power module and which type of audio cable conects the two, 4pin Din or Phono! Therfore any noise is comming from the unit before the 405 as the 405 is a fixed gain block working at full binge into the speakers!

The fact that you get a crack from the amp when switching on seems to me to indicate a DC offset. What sort of noise does it make when you switch off?
I still think, in your shoes I would either take the thing to Huntingdon, which you can still arrange to do. Or courier the thing with a brief explanation of what the fault's are and you will receive back something which works as it did upon leaving the factory!
I have no connection with Quad other than being a very satisfied customer over a period of 35 years.

good luck
mike