mulish

resembling or likened to a mule in being stubborn.
"a mulish expression"
Similar:
obstinate
stubborn
stubborn as a mule
pig-headed
recalcitrant
refractory
intransigent
intractable
unyielding
inflexible
unbending
bull-headed
stiff-necked

Virtuosos can be surprisingly mulish if their methods are criticized.