When using sheep I've found it best to stay clear of the Merino, English Leicester, Shetland and Awassi breed as they tend to seriously over damp the sound, while the Lleyn, Charollais, Wiltshire, Shropshire and Hampshire tend offer the best compromise in absorption.
My only caveat is that on rainy days it gets a bit gamey and tends to ruin the whole affair.
All the best,
Nonoise