I believe this posting by Thaiman on another thread will answer your question :
Re: which krell to buy the KAV 300xi / KAV 400xi / KAV500i
300i is brutal piece of amp that will do rock and dance better than most, rrp £3000, I sold my a while back for £850. if Slam and attack is your sound it will rock your world.
400Xi is a very different amp to 300i all together, it sound a lot more hifi but not clinical. bags of power, lovely soundstage and imaging. The 400Xi will happily play classical music as well as heavy rock and dance. Get the rest of your system right and this could be the last amp you will ever need. rrp £2795 (2nd hand about £1400 - £1500)
The 500i is base on 300i technology (with some compromise I believe) but it is a 5 channels amp therefore I never heard the model.