In regards to hardness ratings and absolute quality of metal. You should be looking for: 1020 or 1025 Double Forged of J.I.S grade steel.
This is how it works, The more times steel is forged the harder and more compacted the molecular structure gets. The hardness rating increases meaning the ball will spin a tad more but the club will lose its softer feel.
Mizuno and Endo both Double Forge, It is more wear and tear on the machinery when a heavier press is used and only twice vs 4x.
Also looking at quality, you want a company that can do very sharp and pointy edges without the use of CNC milling. If you look at the EPON Type S, or JPX Forged and other irons made by Endo the corners are crisp and end to end.
Also keep in mind softness also depends on the sole and head design, the loft, of the club and its materials used.