There is one type of 2a3 to be aware of and this is the JJ 2a3-40. This unique version will handle more power, just like a 300b. So if you want a bit more out of your 2a3 this could be the way you do it. The power supply has to be able to take the extra current but if it can you're in business. I haven't heard a pair but reports are that the sound is good. I use EH 2a3s which I like and recommend.
Interesting comparison between 2a3 and 300b here. You can hear that the 2a3 is good on tone and detail. The 300b smooths out the sound and loses the leading edge of acoustic instruments. Euphonic but not for me.