Net Audio basically re-design the boards I gather and I'm deeply suspicious of one or two of their re-designs... Dada keep the original, but just update caps and so on which would be pretty knackered by now anyway. Not sure the 405 op-amps need to be as way-out as the ones suggested as too wide a bandwidth can apparently cause oscillations the crude originals miss. The OPA134 seems to be a good more modern compromise and still goes out to a few MHz.
Check earthing again - A 405 isn't designed to be earthed directly to the mains as originally conceived. The case is tied to the signal returns and the amp is usually 'earthed' to the preamp, as is the 303 I have.
Dada have a great friendly forum for just these issues. Well worth asking.
Tear down these walls; Cut the ties that held me; Crying out at the top of my voice; Tell me now if you can hear me.....................