I would be very careful about listening to vendor opinions. They do sometimes have their own agenda and they also do not necessarily know exactly what they are talking about.
It is possible that the Arcam may be better than the Krell. I would hesitate to believe that because the Arcam is an AVR (with built in amplifiers). It also does not have balanced XLR outputs. I will believe that the Arcam sounds very good, however.
The cost of Arcam AVR is very close to cost of Krell Foundation. The economy of scale tells me that better components would be used in the Krell (krell does not have any internal amplifiers).
I don't know the "sonic signature" of the Arcam, so I can't comment on how it would match up with MC303. I do think that the Marantz would be too warm/slow for the MC303, but that might be what you want.