Without even looking at the schematic (actually having no idea What a Hampton preamp is) , I can tell you that E80CC and ECC88 are very different valves, only the pinout is the same. How did you get the idea that they are interchangeable? Maybe you meant E88CC? This would be compatible, ie same valve, different specs (long-life, microphony, noise etc). If you plan to swap E88CC for ECC88, you are not going to do any harm. The sonics will likely change a little (possibly for the better), but this is subjective. Try and see what you like best.