AB_LM 

from TBProAudio

There is no harm in listening to A/B comparisons of signal quality before an after an effects chain at exactly the same volume.
 
All these plugins basically consist of a series of plugins that are set up in a certain configuration. And some sort of rms measuring device.  It is possible to set something yourself using the signal flow of your DAW and a quality VU meter in order to create two signals of equal loudness.
 
It does remove the louder is better thing and leaves behind only the other stuff you have done to a master or a buss or a track.  And in many cases the signal will sound worse meaning you have made it worse! This can be very revealing. What you are actually doing to a mix.  It is important to listen to the transients carefully in a mix and you can do this when the two sources are at a perfect level match.  It will be the compressors that are screwing up transients say.  The attack setting brings them back in.  Stuff like that.  Make sure that EQ is just acting in the right part of the spectrum.....
 
Ian's plugin is rather expensive for providing this kind of thing. TB Pro Audio AB_LM is cheaper and from what I can see does the very same thing. There may be cheaper alternatives again. 

MCompare is $30.99 at Audiodeluxe today with group code FEB2018.  Just saying.  

Thanks all, I appreciate everyone’s input into this.
 
I decided to follow my own instincts having read and watched numerous articles and videos, and invested in the MeterPlugs Perception. I don’t doubt that the others mentioned here could do a good job, and they are cheaper, but I felt this could really make a difference to me, and decided on (perhaps) the Rolls Royce version.
 
I have a decent acoustically treated studio, and am enjoying mixing, production and mastering more and more. For me, getting away from the louder is better illusion is a big deal. Being able to make level playing field judgements on effects and processors is a big deal. Being able to A-B seamlessly with one click, make adjustments to the signal chain and do it again and without worrying about manually changing levels is huge.
 
One of the reviews I read in my research was this one by Hugh Robjohns. Now unlike me, Hugh is a real expert and I respect his opinion enormously, but one of the telling statements he makes is “Perception is recommended without reservation for anyone involved in mastering at any level, and I envisage using it routinely as a standard part of my plug‑in chains whenever I am working on mastering projects myself.” I.e., even for non-experts like me, he believes it is a worthwhile investment. Indeed, I think that it is especially helpful for mere mortals, precisely because we don’t have years and years of mastering experience behind us.