Ian Shepherd's Perception plugin

This can be useful.
 
https://www.samplemagic.c...g-the-magic-ab-plugin/

Melda MCompare. Haven't compared then feature for feature but it seems to do a lot, and Melda stuff is generally great. And it's on sale right now!

http://www.meldaproduction.com/MCompare

You can sometimes get them even cheaper at Audiodeluxe or maybe other discounters...

Thanks for your input so far. There is an interesting debate between Ian Sheppard and the MCompare developer here 
 
I am certainly no professional David, but I think this type of plug could be hugely valuable for an enthusiast like me who has often been fooled into thinking louder = better. 
 
Any further comments more than welcome! 

Another vote for MCompare.

One thing to bear in mind going forward is that plugins with complex internal routings can just be bypassed to achieve the OP. I have shied away from the one-off plugins recently, since saving complex presets once makes life easier, and Melda's stuff is free upgrades for life and each purchase builds towards the "whole shebang." They are also nicely scripted so you can dive deeper into the guts of them as one feels comfortable.

and if you are mastering,
you'd better know what you are doing about level matching, as well as tone matching.
otherwise, you are barking up a tree.
 

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. 

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.