Brm1, I would suggest that you contact a manufacturer that makes both 300B and 845 SET amp models and get his/her opinion on where the differences lay. One such person would be Joe Fratus in the USA and owner of Art Audio who makes several 845 models and at least one 300B model too.
The 845 is a bright shining tube, like a 100 watt lightbulb, and runs very hot so these characteristics may be off putting to some. By comparison the 300B is much less bright and hot but both tubes are usually run in pure Class A so expect some heat. Also, the 845 tube is an old radio re/transmisson tube so it doesn't have the frequency range/extension that other tubes have and that is why Gordon the owner of the well respected company Wavelength won't design using the 845 tube, or at least that is what he told me.
Another tube you may want to explore is the great KR Audio PX25 tube that is amazing. Ask Joe at Art Audio about it.
Hope this helps some.
Kevin
The 845 is a bright shining tube, like a 100 watt lightbulb, and runs very hot so these characteristics may be off putting to some. By comparison the 300B is much less bright and hot but both tubes are usually run in pure Class A so expect some heat. Also, the 845 tube is an old radio re/transmisson tube so it doesn't have the frequency range/extension that other tubes have and that is why Gordon the owner of the well respected company Wavelength won't design using the 845 tube, or at least that is what he told me.
Another tube you may want to explore is the great KR Audio PX25 tube that is amazing. Ask Joe at Art Audio about it.
Hope this helps some.
Kevin