Pay attention to the "frequency response" and "low frequency maximum acoustic output" charts. In step down, mode the cabs do play lower, but you lose sensitivity from about 50-150hz, so you will need more amplifier power to achieve the same SPL. Then, in order to get flat frequency response, you need to use equalization that provides 6dB of boost with a Q of 2 at 45hz, and a rolloff of 12dB per octave at lower frequencies. Also, per the builders plan, you lose 2-3db of max output between 60-90hz.
If you are using a 50hz HPF, I believe you are better off using the standard tuning, rather than step down. The only potential advantage I could possibly imagine for continuing to use step down mode with the 50hz HPF, is it may improve the phase and group delay characteristics of the system; but that's just a guess.
Keep in mind, the continuous thermal power handling of the TL606 with an EVM15L is 100W.