Michael,
I do agree to your point that the designer has control over the entire design in discrete circuitry, but IC designs offer more flexibility. Given the right bias, the 2N5551s do have very low noise and low distortion to boot. The reasons why I used these devices, not common to audio, is that I saw them used in the Stochino Ultra-Fast Amplifier design and I have made this amplifier myself and know how very good it sounds (hence, I had many surplus devices) and I couldn't find anything (read available) close to the MPSA18. I ran extensive simulations with Electronics Workbench and found that devices like BC550C etc., exhibited more distortion than the 5551. Therefore, I did not regret the choice, though I didn't have one.
I have pitched this discrete design against NE5534 (Reg Williamson's design from Electronics World entited 'Simple but Sound' with an active volume control), various NE5532 designs, an OPA604 design (a very good one on its own rights), Bride of Zen and another discrete Class A design using MAT02 and MAT03 matched devices for the input differential pair (Elektor design).
In all of the comparisons, the discrete opamp design sounded right (that is added the least of its own signature). It did not add any warmth or glow, but was not sterile either. It did sound a bit lean (I don't mean not enough body to the music) as compared to a good tube design (which makes the sound more rounded) but for the simple circuitry, I felt that its neutrality and fine detailing and resolution were indeed outstanding.
I have also checked this on simulations against the Low Memory Distortion design that I obtained through a link from here. That seems to be a better design since on a square wave test for 10mS, I find that the simple design's trace begins to move down the scope-screen whereas the latter trace remains on the same horizontal axis. This again is a fully discrete design. But with this circuit I found that it is not easy to change the feedback network since the circuit stops operating, showing errors with the FETs in the input section. (Right, about the flexibility aspect).
I do however, agree that there might be better OpAmps like the ones that you have cited or perhaps, the OPA627 etc., which might sound just as good or better.
[Edited by Samuel Jayaraj on 06-21-2001 at 08:02 AM]