Could use some help with a tough match

Though I have no first hand expereince with them; Audio Research and Sonic Frontiers are reputed to work well with Threshold. Again, I don't have first hand experience, but, I suspect some lower impedance output conrad-johnson pre's would work, and you wouldn't be paying extra for unused balanced circuitry with c-j.

FYI (and to the best of my knowledge), the earlier single ended Threshold amps are cap coupled. The later e series fully balanced Threshold amps are DC coupled. Some earler Threshold amps were upgraded to "e" series with balanced inputs, but are still cap coupled. Most tube pres are not DC coupled (the only ones I'm aware of are Atmasphere and Sonic Frontiers). Adding balanced inputs to your Threshold amps might help with long cable runs, but will other wise just add needless complexity. You might have more (though not necessarily better) tube pre options sticking with your single ended Threshold amp. On the other hand upgrading to newer fully balanced Threshold amps might offer better tube pre options, albeit at a cost. My previous recommendations for Audio Research and Sonic Frontiers comes from an ex Threshold tech with lots of expereince and feed back from Threshold customers. The c-j thought just seems to make sense. Why pay for expensive fully balanced operation if you won't use it. The really high end c-j pres output impedance might be cause for concern though.