could a different power cord cure sibilance?

You might want to let the new phono pre breakin for about 300+ hours before you change anything. Every time you try different tubes you have to give them time to breakin. Even if you put the old ones back in after a few days again they still have to breakin. Location to other equipment can also cause issues you discribe, because it has a high gain stage. Are the wires routed the same way, the ones coming from the arm to the phono pre could be picking up external noise.

Thanks for all the advice. I think Tvad is correct, the new pre revealed pre-existing flaws, particularly in certain recordings; no doubt Mechans is correct that in the end, a Steelhead or something equally adjustable will be required. I am using a low output Grado Reference into a 47K load. The Grado is supposedly insensitive to loading, which is why I didn't initially consider that a likely culprit. Nevertheless, as suggested above, I am trying different loads, starting with 800k. A pain, as resistors have to be soldered in (short term, a little less trouble than making some plugs) and, I assume, burned in for a while. 800k hasn't really solved the problem. I also realigned everything (with a Mint), messed with VTA, and switched power cords; all of that may have helped, a little, but not enough. Scouting various forums, some say the Grados are known for sibilance, but it wasn't clear if that is true of the higher-end ones (although, I was surprised when a distributor referred to the $1200 Grado as a "budget" cartridge. Try explaining that to non-audiophiles!) I will be asking for cartridge recommendations when finances permit.