Your PrimaLuna has 4 and 8 ohm taps for your speaker cables. Try them both and see if it makes a difference.
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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,
"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."
Since this will be used only for the low frequency signals going to the sub, isn't there a solid state solution that would work? People seem to buy these tube buffers for the sound of the tubes themselves. That's not the case here.
I'm using an NHT X2 active crossover on the subs.It has a high pass output but when I use it the KEF's sound less detailed. A better active crossover would probably cure this too. Bryston maybe."
I wasn't exactly clear reading through your original post yesterday. Are you going from the pre outs on your AI directly to your Primaluna amp? Or are you going from the AI to the high pass input on the NHT, and then to the Primaluna? If you are going through the NHT, you have to consider that xover will most likely undo any sonic benefits your preamp has to offer. I assume that if you just leave everything involving the subs off, and use the AI, PL and the Kef's only, you get good sound quality?
ZD, yes, a suitably chosen solid state buffer will of course do the job well, and perhaps (depending on the specific models being compared) better than a tube-based buffer. But a tube-based design that is intended to be used as a buffer perhaps stands a greater chance of providing an optimal combination of high input impedance and low output impedance, especially the high input impedance part.
Burson Audio used to make a well regarded solid state buffer, the AB-160, which may be findable pre-owned. I am not specifically aware of any other high quality equivalents that are currently produced, but there certainly may be some.
And hey, perhaps a good solution would be to simply buy a used minimalist/no frills solid state or tube line stage preamp, which provides high input impedance and low output impedance, and insert it between the existing preamp and the NHT crossover.
On paper, at least, the Silk Audio buffer and the similar Yaqin model look like they would probably be reasonable choices. Although I do have a bit of concern about the lowish 60 db A-weighted signal-to-noise spec on the Silk unit. And I would suspect that the Yaqin is no better in that regard, with its stated 75 db performance (no weighting specified) just being better on the basis of "specmanship."
Normally I don't attach any significance to signal-to-noise specs, because they can be defined in numerous different ways that are usually not indicated. But in this case the 60 db A-weighted number is low enough to raise a bit of a red flag, although it's quite possible that in a subwoofer application it won't matter.
Also, I note that there is no spec on output impedance. Which I would assume is low at mid and high frequencies, but depending on the value of the coupling capacitor that is most likely used at its output could conceivably rise to unacceptably high values in the deep bass region, which is what is important in this case.
Good luck. Regards,