Giovanni, I see that your subwoofer's crossover goes between your preamp and the power amps. It has a low pass output below 110 Hz and a high pass output for the main speakers (from the Gradient page).
So, I was wrong about paralelled amps loading down the preamp down to 9K...and Y adapters.
Anyway, I suggest that you ask Gradient or the distributor what is the crossover's input impedance.
If it is lower than 20 K ohms, most tube preamps will not drive it properly. In that case you need to look for a tube preamp with very low output impedance, lower than 500 ohms.
Any decent tube preamp will work wonderfully with a 50K load, but very few match well with a 10K load, typical of solid state devices.
Preamps with "super tubes" do have low output impedance: BAT, ARC come to mind.
The EAR 868 may work, if it is transformer coupled design as the EAR 912. Ask EAR about output impedance.
You will have decent bass if the crossover input impedance divided by the preamp output impedance equals 10 or more.
I suggest a factor of 20 because most tube preamps are limited in the bass by the size of their output coupling caps. The manufacturer may quote (for example) 500 ohms output impedance at 1 Khz, but it is much higher in the bass range, because the output cap's reactance increases at low frequencies.
Good luck
