Vandersteen 5?

Ask the owner if you can open up one of the M5-HP cross-overs (just 4 screws on the side with the label). If they have the Duracell battery in them, it will have a date on it in Sharpie when they need to be sent back in and replaced. If you have the serial numbers of the Speakers, this will tell you how old the speakers are. If they are in good condition and at a reasonable price, I don't think you can go wrong with them. The sub amps just don't go bad unless they have been abused. One nice thing about the Model 5, they can be upgraded or repaired for as long as you own them. Vandersteen doesn't come out with a new model ever 6 months because of what a marketing department tells them to do. The line has been stable since forever and parts are and will be available. The tweeter in the B&W seems harsh to me and I personally would prefer the Model 5 to them. Plus most of the money in the B&Ws goes into the cabinet where most of the money in the Vandersteen goes into the drivers. Which in my minds means more of my hard earned dollars are going into the sound then into the cabinet department at B&W.

The 5's are a great speaker, and would mate well with your Krell.

If you did buy them, the first thing you should do is to replace all the batteries.
There are three inside each speaker and one inside the M5-HP, all 9 volts. Don't
just take the seller's words for it unless he has prove that batteries are still fresh.
If they aren't fresh, the 5 sounds like crap. Also, replace them with lithium
batteries. They last a lot longer.

I own a pair that I bought used. They are superb sounding with Audio Research electronics and do not require a powerful amp. But the fact that they are upgradeable is irrelevant. I would have to pay several times what I paid for the 5s to have them upgraded. If you want the 5As it's much cheaper just to buy them used. (I haven't heard the most recent 5a Carbons.) In summary, I have no desire to upgrade and I plan to keep these for a long time.

While the Vandersteen 5 are great speakers, the Krell 400xi is no where close to the musicality of the class A Krell amps. In fact 400xi is a wannabe and highly compromised. Krell wanted an entry level player which doesnt sound harsh and thats all the 400xi is. I am saying all this because you are getting into much higher league with your speakers and this amp has every potential to disappoint you.