I sourced my tubes from Tube Amp Doctor when I owned Ref 6. If you can swing it, try the ultra rare Reflector 6H30n-DP (made before end of 1992) tubes that includes gold grids. As far 6550, I highly recommend trying SVETLANA Winged "C" 6550C is an amazing rectifier. It has very controlled sound - nothing offensive can be found in any part of the frequency response. Also very consistent in detail and separation, with clean highs and exceptional balance throughout.
https://www.tubeampdoctor.com/en/6n30p-dr/6h30n-dp
https://www.tubeampdoctor.com/en/sed-6550c-svetlana-c/russia?number=NN281