You will not get the same sound out of the KAV300r as you will out of the KSA100. The 300 R is a Krell budget component and in my opinion is not much better than many other similar types of components (Rotel, etc. . .). The KSA is a lot heavier because it has a properly designed power supply. With an amp, the power supply is one of, if not, the main component in terms of delivering performance.
As to the AR being better than the Krell, this is subjective. Secondly, keep in mind if you go with the AR you will need to replace tubes on a fairly fixed schedule (to be safe, lets say every 2 years). You better get the tubes from AR (all matched, etc. . . you could read a lot of the "up-in-smoke" stories about these amps and not using properly matched tubes). Replacing these tubes, I think, is currently about $700-800 + shipping the unit to and from AR (Minnesota). It doesn't sound like you want to set this type of budget for yourself and you should be aware of this upfront. Also, if you are getting the AR amp at a very good price, you can expect to have to replace the tubes right away, so factor that into what you are paying.
That being said, the AR tubed amps (several models) are great sounding amps. I am not suggesting in any way that they are not. They are also fairly reliable, assuming you maintain them properly!