I can only make a guess, but perhaps the 6550 has a slightly higher gain than the original power supply tube. In both Ref 3's I listened to, all with original layout, one powering a Rowland 302, the second a Theta Dreadnaught, I had to raise the volume to 70 and 76 respectively to obtained the realistic sound level I am used to with my old LS2B with volume at approx 43%. Not a big deal though, because above 78 volume became too loud on the Ref 3 anyway.
