Wondering the same thing, but I'm not sure if 45 is considered superior to 2A3 by a general consensus. While everyone seems to agree that the 45 has a superior, sweeter treble, 2A3 is supposedly the "warmer" tube of the two. Maybe 2A3 could be a better choice for brighter headphones like HD800(S), and 45 for something like HD650? On a side note,. it is my understanding that ECBA could be custom built for 2A3/45 use instead of the default 300B (although the majority of people opt the latter). Can a single Studio unit accommodate both 2A3 and 45? Or does it need to be built around 2A3 or 45 only, so that tubes are not interchangeable in a single unit?