Actually, in TSFS, the Excelsior
just had one thingie, that looked a lot like the one on the Enterprise.
The switch to two smaller ones might have been (and for my money, is) a downgrade of sorts.
I used to subscribe to the warp field stabilizer theory, but I don't know that it holds water anymore. I once posited that the crystal on the NX-01 warp field stabilizer actually wasn't the stabilizer at all, but a crystal for converting warp plasma for those impulse rockets at the back.
Also, Mr. Probert apparently refers to the one on his Ambassador
as the fusion reactor, so it's possible his opinion on what that is has changed over the years? A fusion reactor might make the most sense, especially given the toroid shape.