Matching tube pre imdedance to SS amp?

Bought an ATI 1502 amp and running it with Ohm Walsh speakers and a SS preamp from SAE. Interested in running a tube preamp with it but per the manufacturer I need to be careful about "matching the preamp impedance to the amp"? Can anyone translate that? I have a tube pre that I ran with my old SAE power amp and it sounded good. But I may have been lucky...so what am I looking for to match this up? What happens if they are not matched appropriately?Help?