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

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??

At this price range I would look at something like a McCormick 500, there is one on here now for $3495. Much newer and .IN MY OPINION, would sound better.