Are there options to the Hardy 990c like the SC25s, Forsells, Avedis that would work as well without much modification? Or was the circuit optimized for the 990c?
DUH . . . read the J99 page on the website . . . yes you can substitute! Sorry . . . now anyone who has tried different opamps . . . like the SCA, Avedis or Forsell care to comment if these are different from the Hardys.