Referring to the rev 1.4 schematic at the top of this thread, Q2 & Q3 form a Muamp. Muamps are desirable because they provide high gain, large output swing and symmetric clipping.
Let's analyze the circuit. Given that Q4's base current is very very small, we can say that Q2 & Q3 have the same drain current. With me so far? Good. When a Muamp is working properly, the top JFET (Q2) controls the average voltage at the junction of Q2-S & Q3-D. The bottom JFET (Q3) controls the drain current of both JFETs. Saturation (clipping) occurs when Q3 is driven hard enough that the Vds (drain-source voltage) on one JFET or the other goes to zero. If the circuit is well-balanced, the clipping is symmetric or nearly so. There is another situation where the Muamp will saturate: if either JFET drain current reaches Idss. We usually do not want this to happen because it reduces the headroom and the clipping is asymmetric and much more harsh. J201s have a very low Idss and Vp. At high GAIN settings, it is easy to drive Q3's Vgs positive (gate voltage higher than source voltage). When this happens, Q3's drain current reaches Idss. Think about what happens if Q2's Idss is lower than Q3's. Q2 will saturate long before Q2's Vds drops to zero. This causes very asymmetric clipping and reduced output. If we select Q2 such that its Idss is greater than Q3's, then Q2's drain current will never reach Idss. It's not so much of a concern with PF5102 or J113 because their Vp and Idss are much higher. Still, best practice is to select the transistors in the Muamp such that the top transistor has a higher Idss. Note that the two JFETs do not necessarily need to be the same part number. Also, R12 & R13 do not need to be equal.
