@wig "I own and have tried all of the tubes mentioned except the Space Tech
Labs and the Sophia Electric Aqua 274B blows them all away; this tube
sounds like a Dac upgrade..."
We need to consider the application for the rectifier. A power amplifier will obviously presents a FAR different load than a DAC.
Aside from that, the 5U4G / 274B is directly heated, with far more voltage drop than the indirectly heated 5AR4. The former have a more romantic, lush, and much looser sound, the 5AR4 has a much tighter, more focused presentation. The performance of these tubes really differ in a yin and yang sort of way