Input impedance issue?

Your PrimaLuna has 4 and 8 ohm taps for your speaker cables. Try them both and see if it makes a difference.

I have and the 4 ohm taps work best.

Put a tube buffer in between your preamp and the NTH crossover. The very high input impedance of the buffer will make it almost invisible to your preamp. I think the Musical Fidelity buffer in the square box has a 1 million ohm input imp. that you can pick up for around $200 or less. I noticed the exact same thing on my pre using both outputs, one to the main amp and the other to a powered sub. The tube buffer was my fix and I didn't get it for the "tube" sound, but rather as an impedance matching device.

I'll second Koestner's comment.

As in many and I suspect most preamps which provide two pairs of outputs, the two sets of output jacks on your preamp are most likely wired directly together just inside the rear panel. So the output stage of the preamp is therefore driving both the NHT crossover and the PrimaLuna power amp. While the PrimaLuna probably has a high input impedance, the NHT's input impedance is only 10K, so their combined input impedance, as seen by the preamp, is probably in the area of 8 or 9K. The output impedance of your preamp is specified as 1.2K, and mostly likely rises to considerably higher values than that at some frequencies. An output impedance that is high in relation to the load impedance, and that ALSO varies significantly as a function of frequency, will result in the kinds of issues you are perceiving.

I'm not sure if the Musical Fidelity buffer (the X-10v3 I believe) is still being made, but if you can find one it would be a good solution. Also, another member here who had a similar issue has reported an excellent experience having a tube buffer custom made for him by Tom Tutay of Transition Audio Design, in Florida I believe.

Good luck. Regards,
-- Al

Thank you! The hunt begins!