This listing is for MMSSLCHPMS Compressor (with MS Mod Option) only. Use description below as a general reference. Proudly from our beginning, we at MANLEY LABS have been closely involved with numerous mastering facilities around the world. For these most demanding engineers in our industry, we have created specialized products such as our all-tube tape repro head amps, digital converters, high-powered vacuum tube monitor amplifiers, through to custom monitoring consoles, all the way up to complete facility design and installation. The Mastering Version of our Variable Mu® Limiter Compressor incorporating detented and logable steps built with 1% metal film resistors on sealed gold-contact Grayhill switches is available. NO, we will not mod a "normal" unit to the Mastering Version. The High Pass Side Chain mod comes stock on all Manley Variable Mu limiter/compressors since 12/2009, on both regular and mastering versions. This mod adds two switches to the front panel, one for each channel, so that when engaged, the side chain will not respond to frequencies lower than 100Hz. (We standardly use 100Hz as the -3dB point. Other frequencies can be custom ordered.) This HP SC Mod can be used with music with heavy bass lines or bass-heavy mixes where you don't want the bass driving the whole action of the compressor. The MS Mod turns the left channel into “middle” and the right into “sides” to compress each phase component independently. For example, squash the vocal, kick, snare, bass (in-phase info) while keeping the out-of-phase program intact to make the stereo image super-wide. Do the opposite to make the result more mono and radio-friendly.