I want to explain a little on the OPS stage in my amp. I use the Folded 3EF as shown in the drawing where the top 3EF is PNP-NPN-NPN, bottom is NPN-PNP-PNP. The advantages are: 1) Output can swing closer to rail. The OPS can swing to 1Vbe from both rails instead of like 4Vbe from both rails. That's 3Vbe or at least 1.7V gain in voltage swing on each side OR 3.4V total swing advantage. For Class A or high bias Class AB that dissipate a lot of power, you need to use lower rail voltages. This can make quite a bit of difference if you can swing 1.7 closer to the rail. 2) As circled in green, the pairs are screwed together on a separate heatsink, the variation of Vbe of the two transistor due to temperature change literally cancel out. The bias spreader only has to compensate for the Vbe of the output transistors. This is a big advantage over the convention 3EF where it's NPN-NPN-NPN on top and PNP-PNP-PNP at the bottom where you have to compensate for all 6 transistors. I have two amp with this design and the bias vary about 15% from stone cold to burning hot( driving load for period of time). In normal operation, the bias drift is much lower. So this amp does not need a lot of warmup. 3) Folded 3EF has natural output short circuit current limiting. I design with tail current of the first transistor( pre-driver) of only 10mA, if you calculate assume hfe of the follower two stages are 50 each transistor. The total current gain of the driver and output transistor are 50 X 50=2500. So the current limit of the output DC current is 10mA X2500=25A. It is high current, but not as high if you have the 9 output pair of regular 3EF that can pump like 100A!!!! If you look at the diagram and follow the voltage starting with output equals 0V. The Vbe of the big output transistors and driver transistors are typically 0.55V only. If you follow the voltage written in red, you'll see the voltage across the bias spreader is 2.2V-Vbe1 and Vbe2. Small transistors normally has higher Vbe like 0.7V, so the bias spreader voltage is 2.2-2X0.7= 0.8V. Bias spreader needs more voltage to establish a stable voltage, 0.8V is not enough from my experiment. That's the reason I need to choose the pre-drivers with low Vbe. As I said, higher current and bigger transistor has lower Vbe, so I have to use big TO-220 transistors for pre-drivers. that's why it's a lot more complicated than just find another transistor. It affect the thermal stability of the amp. I have enough for my life time of hobby of these transistors, but I did not figure into anyone want to build my amp!!!