Which tense to use?

I can't make up my mind which tense to use in the following sentence:

"If it hadn't been for the remarkable opening passage, I would have assumed that the recital would be / have been a bore.

I've seen both tenses used in similar constructions. Is it safe to assume that both options are correct in this case? If not, is there a logical way of determining which tense should be used? If possible, could you give me some examples?

MarvinTheMartian wrote:

I can't make up my mind which tense to use in the following sentence: "If it hadn't been for the remarkable opening passage, I would have assumed that the recital would be / have been a bore. I've seen both tenses used in similar constructions. Is it safe to assume that both options are correct in this case? If not, is there a logical way of determining which tense should be used? If possible, could you give me some examples?

  We have the pluperfect in the first clause and the conditional perfect in the second [typical type III conditional sentence].  Any verb that follows must not be such as to put the action before that of the other two verbs:  therefore the present of the conditional (would be).