Best used Krell two channel amp?

For those of us who are just now looking at upgrading, there is a dizzying array of Krell amps to choose from. In your opinion, what second hand Krell would you recommend as being dependable and affordable. I would like transparency and effortless power to run my big B&W's. Thanks, Jim
My favorite Krells in the past have been the original KSA 50MkII, followed by the KSA 150/250, along with the KSA80/200, their predecessors. The 50, 80 and 200 are all pure Class A designs, and after warmup can sound quite nice (for solid state); the 150/250 were actually not fully pure Class A in operation, as I recall from the Stereophile review, but heavily enough biased in Class A that they too could drive just about anything out there, and they were pretty nice sounding as well, perhaps a little sweeter and warmer, if possible for a Krell, than the 80/200s. My personal favorite was the 50Mk II, which had a slight sweetness in the highs that the others did not quite match, but that's the oldest model here and used a cooling fan, which is another part that could go in the unit--the other versions have massive heat sinks. All run very hot, and can double as a space heater. After these models (and the Audio Standards), Krell went to a Sustained Plateau Bias system that kept the amp running cooler but in my view lost some of the character, and ultimate power into difficult loads, of the earlier models. The later models have their supporters too, though, as I'm sure you'll find from the answers to your post--I think it's safe to say they may be more accurate in passing the signal, but I always felt they lost a little of the music along the way. My opinion, likely not shared by many others!
I own the Big B&W800's.

Over the years I have owned many Krell amps. Without exception, the newer the model, the better. I think the biggest improvement was with their newest basic technology. the FPB series.

I would aim to try to get the FPB300.

My Krell experience started with a KSA 100s it replaced 2 Aragon 2004's very much better. Moved on to a FPB 300 again a marked improvement in all areas. I was not ready for the differences, when I got a FPB 400cx.

The improvement in the latest version affirms the genius of all the people at Krell.

You should save your money until you can get the latest version. As far as dizzing array statement, there is only the FPB line that should be considered for music.

If you are watching TV that is a different story.
The newer Krell FPB x-series are the most musical amps they have ever made. Some people like the older KSA series, but I have spent hundreds or thousands of hours listening to both, and none of the older amps are as musical as the newest amps. Depending on how much money you have to spend the FPB 400cx is a good choice, if you have the money the FPB 700cx will dominate a big speaker like the B&W's, and let you hear what they can do with the right amps.
How about KAV 250A? I'm planning to get it over KSA80B.

Would it be a good choice?
I would not take a KAV over a KSA any day of the week. I'm not saying the KAV 250A is bad, it surely has the power ratings, BUT the KSA-80b has musicality in spades all over the KAV series.....KAV stands for Krell Audio Video and this seems to be where it is best suited. KSA aka Krell Stereo Amplifier is where the music begins.

I had nothing but older Krell amps at one point, starting with the KSA-100, KSA-80, KMA-160's(mono version of the KSA-80's), KSA-150 & 250, MDA-300's & 500's. I borrowed my dealers KAV-250a for 2 weeks for comparison with the KSA series on hand. Not even in the same league! It would get "in my face" no matter what I listened to. A BIG IF is your speakers? Not knowing what you have is tough to say what will happen. In my honest opinion I truly believe the KSA-80b to be a much better build quality, sound quality and more relaxed sounding amp. Don't let the 80 watt rating fool doubles in power as the ohms are halved and has as much current as some MIG welders. The KAV's do not have this ability.

The easiest way I can describe what happened in my system goes like this: While driving Thiel CS-3.6's which are ruthless of upstream components the KSA-80b filled my large basement/apartment with full rich sound. Switched to the KAV-250a and my Thiels did nothing but "scream at me" unless played at low volumes. KAV always sounded strained no matter what cables I used.

I hope that helped?
I agree with Bryhifi, the KAV series was never intended to be as much a no-compromise design as the KSA/KMA series. It runs in Class AB, not Class A like the 80, and does not have nearly the same ability to drive difficult speaker loads. It was intended to be more user friendly, in that it would fit easily in a rack, didn't run hot, and would have enough power for speakers presenting an easier load, but it lacked the refinement and grunt of the KSA series.
KSA250 KSA250 KSA250. Try to get one cheap so if you dont like it sell it. May want to look into tubes...