When mastering dynamic classical music, the biggest favor you can do yourself is to avoid any form of aggressive processing, be it compression, equalization, limiting, etc. Your stereo image and your signal's peaks should both be retained as much as possible when mastering dynamic classical music.
I usually start a mastering chain by making the lows on the side image mono - but this doesn’t work in a classical mastering session, since the image should be retained. For example, double bass is often expanded into the side image but should be left intact during mastering.
So when using subtractive equalization, be sure you aren’t affecting your stereo image.
This point is almost a given, but classical music should be left as unprocessed as possible to retain the original timbre of the recording. When listening to classical music, appreciators of it want to hear a natural-sounding performance - meaning aggressive processing should be avoided.
This applies to limiting, compression, distortion, stereo imaging, and more.
Low-level compression is a great way to make a master more detailed without affecting the shape of the peaks, but it should be used somewhat carefully since it might increase the noise floor. This will depend on the low-level compressor that you use, but best to use caution.
Classical recordings can already be slightly noisy due to how they’re recorded, so increasing the noise floor isn’t best.
Although aggressive compression should definitely be avoided, some very subtle and slow compression can help tighten the sound and pull everything together. If used thoughtfully, compression can create cohesion, making the instrumentation seem more like a collective performance, something that’s appreciated in this genre.
When I introduce downward compression to classical music, I like to only have about .5 to 1dB of steady compression.
When mastering classical music, it might seem like you should avoid distortion; however, subtle saturation can create a cohesive sound, as well as create harmonics that make the fundamentals more easily perceived. Tape and tube distortion are good options, which add 3rd order, and 2nd order harmonics respectively.
Each saturator is different so get to know the processor you use before trying it out on a classical track.
Odds are, the classical track that you’re mastering is already dynamic enough, but if during mixing too much compression was used, counteract this with multi-band expansion. This way you can control the dynamics in a frequency-specific way, and restore the timbre with the attack and release settings.
Ideally, you won’t need to use expansion, but it can be a good way to repair an over-processed classical track.
When mastering a classical track, -20 LUFS to -16 LUFS is a reasonable range to master.
Retaining a track’s dynamics while trying to achieve a decent loudness can be a challenge, but fortunately, -20 LUFS to -16 LUFS is a somewhat easy range to achieve and requires little limiting.
With that in mind, you’ll never need aggressive limiting when mastering classical music.
If limiting does occur when mastering a classical track, it’s best to make it as natural sounding as possible by utilizing a limiter with transient shaping capabilities. Elephant by Voxengo offers this type of functionality and allows greater control over the master’s timbre than with most limiters.
If you’re using a different limiter, use a release time between 50 and 200ms - which will result in a natural, non-distorted sound.
If your limiter allows for it, it’s a good idea to at least try altering the channel linking of your limiter to see how it’ll affect the stereo image. By altering your transient and sustain linking, you can subtly affect your stereo image in a way that benefits classical music.
Granted, this won’t be the case in every situation, but I’d definitely recommend trying it out.