Fritz Carrera 7 Be and 300B power amp?

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.   

At 87db I'm going to say no. In my experience with 300b amps you need at a min 95db to have enough for dynamic swings. also to consider SET 300b's amps really only work properly in the first half of the power delivery they tend to distort fast at the upper levels of their power range.  if your listening at background levels sure but if you want dynamics and a good range of volume for the normal listener then no.  I ran 94db 8ohm speakers on a 300B amp and if I was honest it wasn't enough. only 8wpc but really only 4-5wpc usable IMO. 

Fritz Carrera 7 Be:

There are at least two links on his website that are articles/reviews of his speakers being used with 8-watt 300B amps .

I have successfully run my 88db Carbon 7 SE mkII’s with a 3.3 watt 2a3 SET amp and never was it close to running out of gas.

Remember, it’s that lack of a traditional power hungry crossover that allows the Fritz speakers to sing on just a few watts. The sensitivity isn’t everything. His speakers are a very benign load wrt impedance and phase shifts.

Give John a call and let us know what he says

With Fritz It’s the true 8ohm nominal impedance spec that makes it possible.  I suspect it will mostly have to do with room size and how loud to be able to go more so than the amp can’t drive the speakers well.