I have both 845 and 300B single-ended amps. My answer is, yes the best 845 can compete with 300B in the midrange. The quality of the driver tube might be more important in the 845 than in the 300B, in getting equal midrange. There will be greater variances from amp to amp in the 300B realm than between the two triode tube types themselves. "Compete" doesn't mean exactly the same. If the two amps pitting 300B v. 845 are built to equal standards of design, circuit execution and build quality, the 300B amp is likely to be perceived as delivering marginally more finesse and dexterity and the 845 more energetic dynamics and projection. But the underlying fundamentals can be quite similar. You just have a few more tube options in 300B for voicing an amp than you have in 845. I will say, however, that if the oft-cited midrange magic of 300B SET is your first priority, you will get it from a cheaper 300B amp than 845 for similar sound. The less expensive 845 amps generally don't have as much resolution, clarity, dimensioning and light-handedness as entry level 300B.
But again, I am describing differences of degree. Both will give you essential SET tone. My 845 and 300B amps are the same rated power -- by virtue of the 300B amps being PSET. The midrange quality between the two is fully competitive, but the 845 sounds more dynamic and energetic, and the 300B amps have a little more ultimate resolving capability, in part influenced by the higher silver content in the 300B amps. The larger qualitative differences are in design and execution more than in the tube types.
Phil