How does Krell KAV 400 XI compare to KAV 500i

NO! The 400xi is not better then the 500i in my opinion.
I had all the Krell integrateds and a few pre powers, except the new FBI integrated.

The 500i makes the 300i, 300il, and 400xi look like toys believe me. Its a different beast all togather.
The size, physical bulk and power is to die for and the GAIN is just unbelievable........a true power house! Very detailed and not for all people I assure you.
Its really a pre and power all squeezed into one big box!
Only advise is: make sure you use the right cables to match your system. I am using Cardas Golden Reference and it works well for my Avalons!

The 500i is quite hard to find, much better sounding than the 300i for example. Everyone I see on A'Gon goes in a day or 2.
The 400ix seems to be a popular tryer, time will tell.

Very good question.

I would call or email Krell. I have always found them to be very honest.

Richard