Audio Space Ref 3.1 KT88 - biasing

I've just got hold of a Ref3.1 Kt88. Great amp, but want to change the KT88's. Haven't got a manual and can't find anything online about the correct bias setting on the VU meter.

The stock tubes are set at '9' on the scale. So my question is: does anybody know the correct setting and what make the 'stock' tubes are?

I have put a request into Audio Space, but no reply yet and as these amps aren't available in Australia, there's no service advisor.

Thanks in advance.