What is the best speakers for

What are the best sounding used speakers that you could get for around $3000.000 for the Krell fpb600 amplifier?
There is no definitive "best" of anything. There are way too many variables involved. The "best" you can hope for is to find something that works well in your system. You can then say it's the "best" for you until you find something you think is better !!!!
Used or new?
And what are the sonics you love?
Do you want hard Rock slam?
Or the perfect strings and female vocals?
Is a great warm midrange the heart and sould of your music?
In order to give any advice, folks really need to know what is important to YOU.
If you have no idea what is most important TO YOU, then you should find out before blowing three grand on speakers someone ELSE might love, but leave you cold.

My recommendation are a set of used Magnepan 3.6s.
Elizabeth has it right, but only if you can have them demo'd when you pick them up, paying cash. I know, shipping in original packing material/boxes will work, sometimes. However, damaged Maggies, whether from the UPS carrier or the seller, are very hard to restore to original spec. A dealer demo would be an even better bet.