By GARY MORROWaNow theJethro Tull-Joe Cocker and Fief t- factor, notsongtheis the importantsolos in it.Ifwood Mac will appear in the Hous- you’re going to play solos, you ton Music Hall tomorrow night. niake them relate to the song, not There will be two performances, let them be an excuse for sayingone at 6 and another at 9:30 p.m.O -byFm a virtus.BASICALLY, ALL THE Tull roup wants is to entertain. QuiteoBILLED AS THE pride of Enland,” Tul! are pioneers. They are one of the first British musical rightly, they believe that their au-groups to break new musical fences want to enjoy themselves.they make a conscious effortSo’Oground and to plant new soundsin contemporary music. Being one to see thiJt their performances ai of the first; naturally they have ]oth musically and visually excit-reaped almost unprecedented com- ing-mercial success.Tull has been together two years.In that time they have had onealbum, “This Was,” which made the number five spot within sevenweeks in England. This was followed by a single, “Living in the Past,” which made number three. “Stand Up,” their latest album, became number one in Egland in less than a week after its release.THEY WORKED HARD for eight months gigging from small clubs to ratty pubs and back again.JETHRO TULL, ONE OF the“Things are changing a bit now. three British groups performingIt’s getting more cerebral,” re- tomorrow night in the Houstonfleets Ian, the leader of the group. Music Hall.