Our Dorroughs, the "loudness monitor", are one of the oldest pieces of gear we have in the studio. A tried and true metering product that I would not want to work without. Simultaneously showing peak and RMS metering our 40 series dorroughs are another investment I have always been glad I made very early on in my career.
RMS metering is the truest way to meter 'loudness" as it's response closely matches the response of the human ear. Peak metering however is still important as it is showing what is truly hitting your equipment and recordings. Sure our Crane Song HEDD and Sequoia [DAW] are showing this more accurately, but the relationship between the 2 metering functions is an important factor.
At about $900 for a pair, it's a solid investment that you will take with you for your entire career in audio and music.