I have a cousin who has been working out with nothing but dumbbells for over a year now, and he has gotten significantly stronger; however, he just switched over to a 7 ft. barbell and his bench using the bar has actually gone down significantly since over a year ago, when he last lifted using the bar. Is this mostly due to neural adaptations or what? And, is there anyway to convert a DB bench into a barbell bench in terms of weight?
I would say his decrease in strength is most likely due to technique. Of course many factors play a role here. Such as, how experienced is he? Did he train with a barbell at any time during the last year? What other type of work did he do with the dumbells. I am not sure what you are asking in the second question but if you want to know if your dumbell bench should be a certain % of your BB bench then: I would say they are two different exercises, even though they are the same movement, and I do not think you should get caught up in worrying about the % of one lift compared to another.