I believe HK's preferred curve is a 10dB slope.
I'd love to see one of these tests done with well designed, modern equipment and speakers such as a pair of REVEL Salon 2's. Then use different DSP's to determine which slopes, if any would be preferred. The speakers used back in the day were these tests were done had crude crossovers, poor off axis response, higher distortion, etc...so it would be interesting to see if those results would still stand.