Welcome to the Carver site!
I have C1, C4000, and CT-7 preamps. The C1 has the BIllD mods and it is dead quiet. It's like it's not even there. Amazing really. I use it in my main rig, with 2 m1.5t's and Infinity Kappa 8 speakers.
The C4000 has been refurbished though it still has some nascent "leakage" into the left channel at zero volume, but sounds great with volume up. It has a Gundry filter installed as part of the Sonic Holography, which I like better than the straight up Sonic Holography. I use it with my RAM285 tube amp and, really, it's my best sounding system overall.
I also have a CT-7 which has not been refurbished at all. It sounds great and is convenient with the remote that comes with it. I use it in my garage setup, as I have it up high and somewhat inconvenient to reach. For some reason, I don't like the phono stage on it as much as the other two.....again, it could just be in need of refurbishment.