I have a Delos and if you're having sibilance issues, it's definitely not the cartridge. As others suggested, do you hear sibilance on all records, or only some? Just to give you an idea, I have a few pressings of "Synchronicity" by The Police and one of the copies is unlistenable because of sibilance, while the other sound great.
If you're having sibilance with vocals on all records, I'd check the VTA to make sure it is in fact neutral, which is actually tricky to get, and if it is, lower it just a bit to see whether you can hear a difference.
However, I suspect your phono preamp is not the best match for the cartridge for a few reasons. The manual for the Electrocompaniet phono preamp states "Gain 1 KHz Moving Coil (source impedance = 10 ohm)," which would be very low if it is in fact input impedance. I have my Delos loaded at 200 Ohms, but I've heard of others who have it loaded anywhere from 100 to 500 Ohms with good results, but 10 ohms is certainly too low when loaded directly into the MC input. When I first got my Delos, I called the Lyra distributor to inquire about the loading, and the person I spoke with said that loading it at 5-10 ohms would "kill it" (his actual words). (It would of course be different for a step-up transformer.)
Further, the gain for your unit is stated as 73 dB, which is WAY too much for the Delos. Your line preamp's gain and your speakers' sensitivity also come into play here, but from my experience, a gain over 60 dB kills this cartridge (my words). At an output voltage of 0.6 mV, the Delos does not need more than 55 - 58 dB of gain at the phono stage level to perform its best. When I switched gain on my phono preamp to 63 dB, the sound became strident, shrill, and overall unpleasant. It did not matter much when I lowered gain on my line preamp; the sound was just not right until I lowered gain on the phono preamp. In fact, it sounded like serious sibilance amplified to an uncomfortable level. At 73 dB, I cannot even imagine how aggressive and un-musical the sound must be. Something to think about.