Who does best SS preamp modifications?

Better to sell what you have and buy what you want. At $2400, the Rowland Capri is very hard to beat.
State a price and ask for recommendations. There are a lot of great used products for sale on here every day.
Two names I would look at, depending on budget are Klyne and Marsh.
You never get your money back when you sell modded stuff.

No need to sell if it sounds the way you like it Angela.

One thing that I would strongly suggest, is to match your preamp to your speakers/amp of choice as well as your personal taste and preferances. It would make perfect sense to call the moder of your choice and ask him for suggestions that best fit your needs. What you get for your money is not only the upgrade but his knowlodge. There is a lot of guys that do mods for living, however not all know what they are doing. Those that do,can be found here and you can read about their work based on those that had some work done on their unites.
Check out Bob Backert thread. Highly recommended.

If it sounded the way she likes, would she be looking to modify it? Maybe, I don't know her, but I suspect the reason behind the question has "dissatisfaction" in it somewhere.

Mod is short for modify. Modify means change. Change means make different. It does not necessarily mean make better.

Often when one looks into this course of action it is in the hope of finding a miracle or a bargain. Generally speaking, you will do far better by applying the "mod" funds to a real upgrade. Unless you have a quantifiable and well-known mod done, you will never retrieve the investment upon resale.
Sad fact.

If, on the other hand, you make a smart purchase on Audiogon, you're chances of reselling the next preamp at no loss are very good indeed.

Pick your poison. You weren't asking for this kind of advice but I hope you think about it anyway.