You can not go wrong with one of the KSA/KMA's..or the newer FPB's. Check various threads here on Audiogon, for specifics. It all depends on how much money you want to spend, and how much power you require.
The Original KSA/KMA's and the MKII series of amps along with the 80 and the 200B were class "A" rated. After that Krell did not rate the amps up to full class "A" power. You can find am awfull lot of info here in the forums.
I would like to take exception on the mis-information that seems to be rampant here on Audiogon, that a capacitor will only last 10 years. This is total bull! There are several factors that determine Capacitor life:
1) amount of heat it has been exposed to.
2) material used in the manufacture (this typically drives price)
3) amount of use
4) when it was manufactured
It is like a tube, you need to have it tested to know if it is bad.
The Caps that Krell has been using since the mid 80's are not cheap caps. These are caps that the Military used, not what you would find in a tv from the 60's, or a cheaply built reciever from the same era.
Buying an older amp is similar to purchasing a used car. It's not the Caps in a unit, You are completely at the mercy of how well the previous owner cared for it, stored it, and used it.
Best wishes