SETs have a limitation of the greater the power, the less bandwidth is obtained from the output transformer. 7 Watts (300b) is a practical upper limit to what can be considered 'hifi' bandwidth. The 2A3, being good for about 4 Watts, allows the designer to have a wider bandwidth and therefore less phase shift (which can be heard in how the sound stage is presented). The reason the type 45 has ascended the throne in the last 20 years is for this very same reason.
A pair of 2A3s in push pull can produce about 16 Watts, making for a more practical amount of power. If the circuit design is good, it will not cede anything to an SET; quite the reverse.