One thing I can help you with now is to suggest you stay away from the Rotel. I tried one myself in my old Adcom setup (which was a GFP555II, not a GFA..) and found it to be too noisy. For instance, when I turned my preamp to the input served by the Rotel, I heard hiss, noise, and static, although at a low level. If I turned the volume up, things got worse. Sorry, I don't remember the model number- I only loaned it from HiFi Sound in Mpls (nice guys!) for the weekend, and that was enough. Went straight to the Jerry Raskin/ the Needle Doctor, which was about 6 miles away, and ended up spending about $300 for an Acurus P10. Not sure if they are made any longer, and while they aren't the "last word" in phono preamps, it's good enough for me. This is not a recommendation- it's just the best I could afford back then, but it may be worth noting that out of all my equipment, this phono preamp is the only thing left untouched in the last few years. Thanks, Jerry!
There's a recommendation- call Jerry Raskin, or at least check his site at www.needledoctor.com.
At the time of that purchase I had a Planar3, which I really liked. In fact, I liked it so much I traded up to a P25 like yours about 2 years ago. I've got three different stlus'- Shure v15, Sumiko Blue Point, and a Blue Point Special, which I change out to suit my moods, and like almost all decent phono preamps, I can change the loading on the P10 to match each particular stlus (more on this below).
As per your cartridge choices, I have no history with either one, but have heard good things about both on a P25. I do, however, know a guy on Audiogon who has experience with a Dynavector, and last I heard was considering the Rega, I think mainly because he figured the Rega stylus was engineered to work perfectly on a Rega table. I'll see if I can get him to say some words on your thread.
Here's another thought. The Rega is a moving magnet design, like my old v15, and I don't know what the Dynavector is, but I found moving coils to be too bright with my Adcom. It wasn't until I bought a Sonic Frontiers Line1 preamp that I could live with moving coils. I only keep the v15 around for a few specific albums.
Also, if you think you will ever want to change out your cartridge/ stylus, be certain you get a pre- that will let you change the loading to suit BOTH MM's and MC's.
Only other thing I can say, and this is from first-hand experience- you might want to shy away from Grado's. I wanted to hear the famous "Grado sound" on my P25 so I borrowed a Grado Sonata from a friend, and I heard the slight rumble that everyone talks about (yes, even with a P25). Once you know it's there, you won't be happy.
Sorry I rambled a bit. I think it was Shakespeare who said, "Brevity is the soul of wit". If this is true, I must be as dumb as a stump.
Best of luck.