http://www.tb-software.com/TBProAudio/ab_lm.html
Requirements: Reaper Daw or Free Reaplugs where you can use it as an JSFX.
First of all thanks a lot to the founders for taking us back to the platform of where it all started.
It's a simple trick which quickly matches your final channel output to the output before your FX chain (or whatever you specify)so that you can A/B without including the loudness factor.
PS: It can even do it in REAL TIME which is always great.
Cheers!

Like I said Reaplugs is an VST FX suite which can be used in any VST compatible DAW.It has an Max For Live like JSFX where we can use this plugin,all instruction are there in both Reaplug's & the plugin's userguide and no you dont need to slave Reaper.