Fritz Carrera 7 Be and 300B power amp?

Some thoughts from my experiences with 300b.

I was curious about 300b tube sound. But they were never going to power my main system. So, instead I chose to have them power my headphone system. Turned out to be a great choice for me. This gave me a great opportunity to experience their fantastic sound. This lead to me completing upgrading my main system to all tubes.

This may be out of your price range but my 300b headphone amp can also be used as an integrated amp and drive speakers. Woo WA5-LE. 
 

To get great tube sound out of speakers like you are considering, a used Audio Research I-50 would be a good choice. Not 300b, but warm and detailed with more power.

I haven't seen the phase or impedance graphs for the BEs ... I'd call Fritz but it seems to cover the lower end:

          Recommended Power: 5 - 200 Watts RMS, without clipping

Fritz will tell you straight up.

Call him.

I’d say most likely but talk to Fritz. 

I also suggest you talk to John, owner of Fritz. 

What is your room size?  I own Fritz Carbon 7s and use them in a 15x12x8' room driven by a PP tube amp with 20wpc. That is more power than is needed for my room.  With this amp the Carbons could fill a much larger room.  That said, my thinking is a 300B SET would be on the border regarding power.