what gear, including cabling, is in your system?
I would say the Krell Phantoms are just the newer generation of the same 222/202/280p preamps. These are how the preamps evolved:
KAV-280p ==> Phantom III
Evo 222 ==> Phantom II (much larger power supply and large analog boards)
Evo 202 ==> Phantom (This is the unit with a separate power supply chassis).
I love the Krell sound. It is very fast, clear, detailed and beautiful. However, it can easily be mis-matched with components (such as silver cabling, bright speakers, etc.).
The Acoustic Research (which I am guessing what you mean by AR) is going to be a completely different sound. I haven't heard, them, but AR is know to be neutral and fast for a tube preamp.