Question Is the Optical I/O on the Reverb4000 compatible with the 8-channel ADAT/Lightpipe I/O on my Audio-interface I only need to send two channels back and forth.

Yes, the optical input on the Reverb4000 is ADAT-compatible. In the I/O-menu when Dig. In Select has been set to OPTICAL, you can then with the Optical Input/Output select which pair of input/output-pairs on the ADAT you wish to use - 1/2, 3/4, 5/6 and 7/8. On the M2000, M3000, Finalizer 96K, FireworX, Gold Channel and the Intonator, you can even select exactly which channel or channels on the ADAT you wish to use.