I tried those two preamps on McIntosh 275 (with Klipsch LaScalar and Velodyne subwoofer) and Linn AV5105 (with Tyler Taylo 7U).
I felt Magnum 99 worked real nice with MC, both the CD and phono. The sound stage was wider, the base was deeper, and more clear and lively sound. PrimaLuna and AE were still nice but they fell behind the Magnum for both CD and phono.
On Linn, the difference was more subtle. The only thing I could really notice to tell was that Magnum was a bit warmer than PrimaLuna/AE. The dynamics and liveliness on MC/Klipsch could not be heard on Linn/Tyler.
More listening would be needed for final judgment, but Magnum seems to work better with tube amp. The price difference (new/old - $5000/$3000 vs $2000/$600) would also be a factor, and with better power amp it can really shine.