The main difference between Keeleys two styles of compressors, one being the older 2 and 4 knob compressors, and the new style compressors GC-2 and Bassist is that the older ones are Compressor/Sustainers and the new ones are Compressor/Limiters.
The Keeley Compressor C2 and C4 – Add Sustain, when you are below threshold. Also, this leads to noise when you aren’t playing at all. The compressor attempts to add “sustain” or volume when your notes are ringing softly. Almost all effect pedal compressors are this way.
The Keeley Bassist and GC-2 Compressor/Limiter are different. These sounds incredibly transparent and natural. They do not add sustain but don’t add noise either, when below threshold!
Both can give you a squashed country sound if high levels of compression or sustain are selected. The old style compressors will give you added sustain. The new compressors simply don’t do anything to your tone when below threshold, SUPER transparent and low noise.