could a different power cord cure sibilance?

Lloydc, if you post the cartridge model you're using, then determining a proper loading range will be fairly easy (assuming an MC cartridge is being used). I have two sets of loading plugs and adapters that might be useful to you depending on the cartridge loading requirements.

As I recall, the Dynavector 20xh is designed to work into a 47k load. Mine does.

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.

08-01-09: Rpfef
As I recall, the Dynavector 20xh is designed to work into a 47k load.

Member Nsgarch taught me to load a cart based on multiplying the cart's output impedance by 9, and then using +/- 50% of the result to produce a range for experimentation.

The 20XH has an output impedance of 150ohms X 9 = 1350. +/-50% of 1350 produces a range of 675ohms to 2025ohms.

I'd start at 1350ohms (or whatever close approximation is easily attained using loading plugs), and work down from there.

This Parallel Resistance Calculator can be used to determine what value loading plug used with the 47kohm factory default will produce the desired impedance.

Basically, you can assume the value of the loading plug will closely approximate the value of the resulting impedance (eg, a 1000ohm plug will produce 979ohms, and a 600ohm plug will produce 592ohms).

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.