Refurbish vs New Amplifier?

To answer one of your questions:

I wonder if at the $2K to $3k range a new amp would surpass the performance of the older Krell?

Yes, in my opinion, especially so if you are buying used. Not knowing what you are using for speakers and pre and source, it would be hard to give recommendations. That is, if you want any.
Bob

I have a KST 100 and it still puts a smile on my face. I know there are better amps but I like to buy and forget about it. I've replaced the caps, set the bias every 10 years or so. Unless you feel your missing something or the quality has slipped you should be set.

recapping every electrolytic caps is just necessary after 15 years and any competent tech can do it for very cheap.

This is interesting. I tend to agree with the Krell advice. Re: older equipment that is working well probably is best left alone. I just had a water pump fail on an old Ford.
had it replaced with new factory pump which proceded to fail. Second new replacement is working fine now.
Caps new or old can fail. So can other parts. I had a tube fail in a CJ preamp one week after plugging it into my system. 
Predicting part failure ...why stop at caps ? Is a personal decision. 
There are no guarantees and apart from rebuilding an older piece of equipment.....(even then.....)
The KST 100 was a well reviewed unit...A comparable Krell (100 Watts) is app $5k.
i really like it . I realize there are less costly amps out there that may sound better
I am intrigued by the Audio Alchemy and a few others  in the end they will have to beat the Krell.

I would keep the Krell and do exactly nothing unless it obviously fails or something starts sounding not quite right. Krell people are correct, this is not the case of preventive approach. It may work just fine for another ten years.
There are always better amps until you reach the top - then just different.