A couple things...
1. Op amps are designed to run with a huge amount of feedback. This on its own will even out the differences between op amps quite a bit. That is, of course, assuming that they're compatible with that circuit - just having the same pinout does not mean that they are an optimal choice. It's not impossible for "better" op amps (e.g. OPA2134s) to not work well in circuits designed for "worse" op amps (e.g. TL072), and don't get me started on using a bipolar input chip in place of a FET input chip - there's no way that won't cause differences because you're throwing the input impedance out of whack.
2. About the only "better for audio than IC" discrete op amp I know of is the 990 design - at least if we're talking just in terms of fidelity. It's both got excellent noise and distortion performance and truckloads of gain on tap, and as such is an excellent choice for mic preamps. The API 2520 and its brethren are also a great design - if you want the distortion characteristics they provide (they're not very clean by modern standards, at all).
1. Op amps are designed to run with a huge amount of feedback. This on its own will even out the differences between op amps quite a bit. That is, of course, assuming that they're compatible with that circuit - just having the same pinout does not mean that they are an optimal choice. It's not impossible for "better" op amps (e.g. OPA2134s) to not work well in circuits designed for "worse" op amps (e.g. TL072), and don't get me started on using a bipolar input chip in place of a FET input chip - there's no way that won't cause differences because you're throwing the input impedance out of whack.
2. About the only "better for audio than IC" discrete op amp I know of is the 990 design - at least if we're talking just in terms of fidelity. It's both got excellent noise and distortion performance and truckloads of gain on tap, and as such is an excellent choice for mic preamps. The API 2520 and its brethren are also a great design - if you want the distortion characteristics they provide (they're not very clean by modern standards, at all).
I'm sorry, but that just isn't true. Feedback linearizes the transfer function - meaning that it brings the output signal closer to the input. If anything, it should improve "microdetail" by lowering distortion. As for altered timbre, again, no - feedback evens out frequency response differences.Speaking with two DAC engineers (and looking at as technical of data I can handle), it seems you can run endless feedback loops and filters to make them perfect, from a SINAD perspective. However, you are actually losing source data/micro details in the process. I am told you also alter timbre.