Krell FPB650m or FPB700cx

Setting up a new system for my in-law and he likes the Krell line of amps.

His pre is the Cary SLP 05, speakers are Wilson Sophia 1.

Now he wants amp that will provide him with extended highs and nice defined bass. Also wanted minimum watts of 500.

There is an opportunity to purchase either of these amps for approx $8000 and $6000 respectively.

His budget max is $8000

Any suggestions or recommendations?

I've been partial to monos and love the Krell line. I don't think you can go wrong either way. A friend has the 650s and they really push his Lat 1000's.
I used the 650Ms with my thiel 7.2s for a few years. They are very good amps, but in comparison to the 700cx i think they can sound a little dark. I preferred this, but many people dont. Both very good amps, win/win choice.
"Also wanted minimum watts of 500."

The Sophia is a fairly efficient speaker. There is no logical reason to need 500 WPC on these speakers
Your krell has more than enough power for the Sophia. Only that Krell's own preamp. can bring out the full potential of the 700cx - or any of the FPB series. Cary, being may give the illusion of sweeter sound, but the sky brightens with a KCT or the newer version such as 202
I'm 100% positive hed get the best possible sound with a Krell evolution 402 and yes i know thats just 400watts but its more than likely going to be enough.

Between the two options listed the 700cx is an easy choice the newer cx series is much better than the c.