Frequency response is NOT the only thing that changes with sample rate changes. Otherwise, we'd still be using 44.1KHz for everything. But 96KHz is now the standard because it sounds cleaner. Most of the distortion is pushed beyond the human hearing range. This is not just an issue of converter quality but the inherant quality of the sample rate itself. Many classical recordings are done either DSD or 192KHz BECAUSE they can hear the difference.
If harmonics are being added at lower volumes but not at higher volumes, then perhaps it's more of an upward expansion rather than a downward compression
OK, you misread me. There are not harmonics being added at lower volumes because there's no saturation. Harmonics are added at HIGH volume because the peaks are getting distorted. It's just hard to hear those added harmonics because the bias causes the high frequencies to sag as the tape is hit harder. Regardless, it's not compression or expansion, it's distortion. Nothing more, nothing less. If you hit a line amp too hard and roll off the high end is that compression? No. In order for it to be compression, the intensity of the waveform regardless of where it is in the cycle will be attenuated because a compressor averages volume changes over time to PREVENT distortion. On the other hand, when you hit tape too hard, it instantly crushes the highest peaks leaving the lower level stuff 90 degrees later alone. That's the difference.
As for tape plugins for DAWs, they do exist and they do not sound like tape. They sound like distorted digital. There's a reason ProTools sounds like ProTools. There's a reason RADAR sounds significantly better than ProTools like there's a reason ADATs and DASHs sound the way they do. Metallica's "Black Album" sounds like it was recorded on a DASH even though it started on 2" and got dumped to DASH for overdubs. The reason, the individual tracks were distorted by a digital medium. That distortion was carried to the final master even though they used an analogue board & so forth. "Brothers in Arms", though also a great recording, sounds brittle & edgy because it was recorded on a DASH. On the other hand, "The Joshua Tree" sounds open and warm because it stayed analogue all the way to the mastered stage. A lot of people won't hear the difference, but I do. That tells me that more than the frequency response changes with sample rate. RADARs run at 96KHz sound pretty good, though not as good as a 2" 16-track. Almost anybody will chose a 1/2" 30I/S master over a 24-bit 96KHz even though the frequency response is very similar.
The thing that we (pro-analogue guys) keep trying to say is things are not always as they seem. Even though modern DACs have gotten very good at trying to fill in massive amounts of missing information, you're still listening to a computer generated guess, not the original. The output may be similar, but it's not the same. Now, the output of a 1/2" tape is not going to be quite the same as the original either, but its distortions are not as harmful. It must follow natural laws of physics and we interperet its distortions as part of the original sound because it by nature follows the physical world. Digital on the other hand, must use rules written by man to artificially construct a new waveform from measurements. Those rules try to emulate the natural physics but there's not not enough information to do it properly. I mean both sample wise and because of our limited knowledge of physics.
Think about this, you can take a plaster mold of your face and make a bust of it. The mold may change shape a little as it dries, it may have some dirt on it, but the final product has a direct physical link to the original.
Now, somebody else wants to make a bust but they use a ruler to CM by CM measure the face and write down those measurements. A different person then later recalls those measurements and makes a bust going by those measurements. Of course he only used CM precision and the details of a human face goes much finer than that. So he rounds off the edges of each 1CM square section leaving the center alone. The final product may look very much like the original, but people would be able to tell that there's something not quite right with it.
I should also add that in all the interviews I've read, all the engineers and producers I've met, all of them who have worked with analogue, like analogue better for its sound quality. They just use digital because it's easy to edit. The only people I really see arguing that digital sounds better are people that haven't logged any major time on some good analogue equipment. I've worked with many people that thought digital was IT until they did a project on some high end analogue gear, some of them even mid-line analogue gear and changed their whole perspective.
