I have now had a chance to dive into the Carvin Quad X. I have provided some photos I noticed that no one has been showing, that elusive bottom board. The unit I picked up had alot of scratchy pots. They are all coming from the cheap pots on the bottom board. I disassembled the entire unit and cleaned all the connections and pots. This helped dramatically but ultimately I would like to replace all the pots on the bottom board. After I do that the unit is ready for another 15 yrs. or so. From what I have read it is a common problem on this amp.
I connected this pre-amp into a Marshall stack right into it’s input. I kept all the setting on the amp neutral but ideally I wish I could have tested this on a plain stereo tube amp. So my first take was quite disastrous. I had a very hard time getting the sound to be fat crisp and properly distorted. I didn’t want to give up on this pre-amp. I removed the unit and tried just guitar and headphones plugged in. Actually this was a great move, it gave me a chance to tweak the knobs such that I can see where all the break points are for the tubes. I found that most of the best settings were when the main volume knobs were between 3-4 or 5-6 and the drives between 6-7 or 7-8. From there I adjusted the stereo out listening for bad distortion (like tonal loss) and lastly the cloaking if using those channels.

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Overall I believe I may be able to coax so great sounds out of this preamp. Understanding the sweet spots is really key and not have any external sound influence other than the Quad X. Once dialed in then I would add effects. Please see below for additional documents.