What is the input impedance of the Classe DR-9 amp

Why not try the obvious? Get in touch with the mfg.

The Model 15, similiar to the DR-9 is 70K into the RCA inputs. The balanced input is probably 1/2, not double as you would expect. This due to the fact that the circuit is not true differential. Either way, any pre-amp will drive it to full output no problem.

Thanks for the input. Tech support at Classe couldn't tell me for sure, saying they thought it probably was 50-100K, similar to the responses here. I might have to resort to finding an oscilloscope and 'tuning' the crossover based on how the amp responds to input waveforms.

The input impedance that I supplied was for the RCA/unbalanced inputs.

According to a review in Stereophile vol 11 no 10 by Arnis Balgalvis, input impedance in stereo mode was 100k and in bridged mono 25k.