Yes, the AVVT 300B is a WE 300B equivalent. I own a pair of the AVVT 300B with the SL-C37 glass, best 300B IMHO. AVVT also made the AV32B which is a high output 300B similar to both KR and the EML 300B XLS.
My understanding is that you can use the higher output AV32B or an 300B XLS tube in a WE300B spec'd amplifier as these tubes can perform at the WE spec'd (filament voltage 5v, filament current 1.3a, plate current 300v, bias 60Ma) and can go up to 500v on the plate with a bias 120Ma without any issues. If your amp is auto biased, you may not get the benefit of the higher output potential of the XLS tubes, but they will work and sound just fine.
My understanding is that you can use the higher output AV32B or an 300B XLS tube in a WE300B spec'd amplifier as these tubes can perform at the WE spec'd (filament voltage 5v, filament current 1.3a, plate current 300v, bias 60Ma) and can go up to 500v on the plate with a bias 120Ma without any issues. If your amp is auto biased, you may not get the benefit of the higher output potential of the XLS tubes, but they will work and sound just fine.