I recently purchase a Carver TFM 15 amp and a Carver C2 preamp off of ebay. I can not seem to get them to work together. I get sound when I play a CD, but I can not control it through the preamp. I can control the volume through the input level knobs on the amp. Why can't I use the preamp to control the volume, base, treble, etc..? Any help would be greatly appreciated.