I wand to design a headphone amplifier based on CFA topology (for some very personal aesthetic reason). There are two basic topologies of its input stage which is technically an V-I amplifier (I-V converter providing voltage gain is usually implemented via two current mirrors "fighting" each other). I want to know why one can be preferred to another. In both cases the design procedure is unclear for me. In particular how to choose resistors' values? I have some clues, but I'm not completely sure.
In case a) \$R1\$ and \$R2\$, I guess, provide some additional Vce room for \$Q3\$ and \$Q4\$.
Case b) should work well (and it does on simulation) without resistors at all. So, I guess, \$R3\$ and \$R4\$ may help to minimize some differences in \$Q5\$ and \$Q6\$ characteristics. But I have no clue how exactly. In case of non zero \$R3\$ and \$R4\$, \$R5\$ and \$R6\$ may serve for biasing purposes.
Shortly speaking, what is the purpose of emitter resistors in both these circuits?
Unfortunately I can't find any article which can explain the procedure. Can you point me to some?