What are the best saturation plugins for adding warmth and character to your mix?

softube FET tube compressor is a great tube saturation compressor that acts like the saturation knob but is a full on compressor, while saturn works to hone in on whatever niche frequencies you'd like to achieve with saturating the signal, and works great on guitar and bass.

I have always considered saturation to be a side-effect of using another effects unit, without downplaying the effect of saturation itself. Unless we are talking about tape saturation on the mix-bus or the mastering process, I always try replacing an existing plugin with a tube analogue of it (mostly compressor or EQ. Sometimes I'll even use the same mastering EQ later in my chain with everything around +3 dB, just to achieve the saturation effect instead of a plain volume adjustment.

The best saturation plugins change their harmonics dynamically with what is being fed to them. They also keep the high end clean and clear with little or no digital artifacts (aliasing, IMD). One plugin with digital noise isn't the worst, but it adds up. These two are good for adding some tube tone, just to sweeten things up. Klanghelm SDRR - wonderful saturation and tone, incredibly controllable. Black Box Analog Design HG-2 - just delicious.

A great analog plugin that features built in tape saturation is the kshmr 'kick' plugin which works great for almost any instrument.

U-He Satin is my choice for sound quality, flexibility, and UI design. There are several plugins in this genre and I'm sure they are all useful in their own way. The goal is to impart that sound quality associated with tape. Satin not only does tape saturation, it also does flange and Dolby A emulation all equally well.

This is one of my favorite types of saturation. The warmth, compression, punchiness...adding subtle tape saturation to everything can really make things feel more cohesive. Softube Tape and Black Rooster Audio's Magnetite have been two of my favorites for interesting harmonics, tone, and adding color. These two also do a great job at keeping away from that "flatness" that plagues digital saturation plugins.

As I've said on the tape-saturation part, try replacing an existing effect with something more colorful. Especially in the case of adding transistor saturation, you can probably push one of the compressors harder to achieve the same effect, but have a beautiful tiny interaction betweeen the effect and the saturation. Feed your FET compressor a weaker signal, bring everything to level after the compressor, and you can achieve an "interactive" saturation which matches your dynamics.

Heavily driven transistors can sound really aggressive. Gently added transistor distortion can add energy and excitement. I've liked Klanghelm SDRR 2, and Black Rooster Audio's VPRE-73. I've heard that Soundtoy's Decapitator overdrives better, which probably would do better getting that aggressive transistor tone, but SDRR 2 and VPRE-73 do an awesome job as they are.

GSat+ by TBProAudio is a new and free plug-in that’s worth mentioning in addition to the ones already there. It has 3 different “tube-based” character modes as well as other useful saturation and gain controls.

There is a massive difference between saturation and distortion/overdrive. Saturation adds subtle harmonics and gentle tone. Overdrive is more similar to a crunchy electric guitar, or a really distorted vocal. Overdrive is much more difficult for plugins to do well, due to limitations and rounding in digital processing. Oversampling can help reduce this issue, depending on the plugin. Some plugins are much worse, others are much better. Try researching aliasing and intermodulation distortion. To make the most out of saturation, make sure you can trust what you hear. A high quality DAC, headphone amp, and transparent headphones are the first step. A treated mixing room and transparent monitors is the second step.