Mike Edwards, 62, was a founding member of ELO and played cello with the group from their first live gig in 1972 until he departed in January 1975.
He quit to become a Buddhist and later changed his name to Deva Pramada because of his religious convictions.
His eccentric cello playing - including plucking the strings with an orange or grapefruit - and bizarre costumes were a major ingredient of ELO.
He played cello on albums ELO II, On the Third Day, The Night The Light Went On (In Long Beach) and Eldorado.
Mr Edwards died instantly when he was hit by the bale which weighed nearly 700lbs.
He was driving along a road when the bale careered down a slope in a field and flipped over a hedge - smashing down onto his roof.
The circular bale is believed to have been in a steeply-sloping field beside the road when it somehow rolled and jumped 12ft to 15ft into traffic.
Police said the accident happened at around 12.30pm on Friday on the A381 between Harbetonford and Halwell in Devon.