I also run a Granite tube output CDP directly into my amplifiers (AtmaSphere M60s).
Now, I have to say that I consider the tube output section of the Granite to actually BE a preamp, so that must be taken into account of my opinion. But, it also has a more traditional fixed, solid state RCA line level output as well. In the latter instance, I still run directly into the Atmas, and use a pair of Endler Attenuators to back off the volume. Depending on the music, my mood, etc., I will use the sunnier, more open tube output, or the more relaxed, liquid, weighty, and full bodied solid state - the ability to switch is a very nice feature of this player.
In no instance, of the preamps I have tried (ARC, Blue Circle BC3, Cary, Joule LA150, and some others), was I ever able to match the purity of running direct. The preamps always changed the sound, and I can see where some of the things they did could be welcome to some, but I never was able to match the resolution, detail, clarity, dynamics, slam, etc., but I without fail noticed more noise, haze, and grunge.
I will say the Behringer DEQ2496 unit I am playing around with now that Eldartford recommended to me has come the closest, by far, to having nothing in the audio chain. And, the ability to compensate for the frequency anomalies my room presents means it probably will be a long term fixture in my system.