Is it good for a Krell to SIT for 3-5yrs

It is not good for any amp and Krells cost a lot to recap. A technician friend was given a Krell by someone who didn't want to spend the money; so far he hasn't either. But each case is different, I was largely inactive in audio for 12 years and some of my amps sat in the box. They are working fine but they are not Krells.

I'm not sure how NOS amps will do as an investment either?

I have heard many horror stories of amps that have sat for extended periods , I think you should look for an amp that has been used regularly . If you find one at a honey of a price , you may want to talk to Krell first and make sure it can be recapped and at what price .
Happy Listening

Funny you should ask...I had a Krell CD-DSP that was updated by Krell to Mark 2 status. I packed it in the original box and stored it in the garage for a couple of
years. When I unpacked it, it only worked as a transport.
I sold it to someone who found the problem. One of the
chips was rusted and needed to be replaced. My advice would be to store high-end items where temperature variations are at a minimum.

Well I have been in touch with the 300s amps for sale and one in NY wants $3500 and it has never been serviced and sat for 5yrs..he wont budge
the other was $3600(no box) and sat for 3yrs in plastic untouched??