Huh, I`m sorry to hear that - what load and operating conditions you used? My pair of E180F "Philips" are (for my ears) a bit better sounding then "Siemens" D3a I have in the same circuit - CCS load, bypassed Rk=226 Ohms, 10-11mA. E180F has lower ยต - maybe it has something to do with it? Did you try it with bypassed cathode resistor (you used anode choke load if I`m correct)? What are your Ua/Ia/Uk with 6688? Can you describe "lifeless" in more details?