Your goal will be to keep the soil as cool as possible. Leave the soil bare through the first half of January to expose it to as much cold as possible, and then heavily mulch to keep the cold in and the sun out. Planting deep will mean later emergence, but there is a trade off: the deeper you go, the poorer the water drainage is and the uglier the soil gets (in several ways). Remember too, that the soil must be good for the roots to grow well below where you place the bulb.
I think this all makes it pretty unlikely that any lily will do well at a 18 inch depth. Meanwhile, maybe invest in some big buckets to have on hand to cover them if nights get too cold. And no watering - that will speed the growth.
I'll betcha Lorn has done some experimenting with trumpet planting depth, but he has that super great soil already.
