I humbly disagree with Oneobgyn regarding tube rolling with the Ref 3. That said, there is only one alternative to the stock 6H30's, which are milspec early '90's Russian 6H30-DR's. My experience is that those tubes do enhance imaging and dimensionality of the sound of the Ref 3; not dramatically perhaps, but noticeably. The 6550 in the power supply is another story altogether. I replaced that with an early 60's Tung Sol solid gray plate 6550 and doing so transformed my whole system. There was an immediate, dramatic improvement almost across the board with every source. Above all, there was a huge improvement in the continuousness and density of the sound (the opposite of grain.) I would call it "liquid", and it is a quality I have not heard in any ARC gear since ARC had to stop using the original, now NOS, Siemens and Phillips tubes back in the '80's.
