Matched Pairs of tubes are special order directly from Emission Labs. Typical delivery is within 2-4 weeks.
The classic 45 was designed for speaker systems needing less than 2x 2Watt of RMS power. Such speakers exist, but many other still excellent speaker systems, require up to 2x 4 Watt output power. Here is where the 45B comes in. The 45B can be used such to keep the sound character of the 45 tube, but do so at higher power. For this, some carefully chosen operating points are recommended below. These will all come near to the original 45 sound, in terms of harmonics development and damping factor, using similar transformer impedance too. For this reason you will see comparable distortion figures as with the genuine working points, only with the 45B that will achieved at higher output power. Additionally, some 45B working points are recommended with relatively high transformer impedance of 9k2, which aim for lowest possible distortion, lower as the classical 45.
The 45B tube will NOT give higher output power when the amplifier is not modified. However the 45B can be used normally instead of a standard 45, and will behave identical. So the modification of the amplifier can be done at a later time if required.
Read More Here: http://www.emissionlabs.com/datasheets/EML45B.htm