Diamond Cut Milennium's paragraphic EQ has presets for the NAB playback curve as well as a reverse NAB curve. I take the original double-speed file, apply the reverse NAB curve to it, then slow it down with DartPro98, then back to Diamond Cut and reapply the NAB curve to the slowed down file. Aside from the sample rate issues, does anyone see a problem with this method?
I forget and my copy of DC is at home...but the NAB curve is speed-specific, so you need to use the 3.75 curve for the inverse curve and the 1-7/8 for the slow-speed curve.
Just watch headroom and noise floor which I suspect won't be a problem, but I did do a Christopher Isherwood tape for the Huntington Library that had a surprisingly low noise floor at 1-7/8 in/s. I transferred that in real time using a Sony APR-5003V at 3.75 in/s and a -50% varispeed. I have the 3rd "tape type" EQ for this speed set up using a 1.88 in/s test tape rather than the 3.75 in/s test tape.