Tune down half step, play all songs in E, which is what was being done on those recordings. Did we solve the whole E/Eb thing? Maybe, but now you need to seriously think about drop D.
You CAN play ROTM and Tool and Creed on a 5 string, but you'll really want that drop D tuning on the 4 string. But here's your issue, if the problem solving tuning above is your way forward, you're going to be actually tuning down to C# and you'll be getting kinda floppy. But...your singer is now used to the 1/2 step down which is understandable. He's always been a taker. But his problems are your opportunities.
You'll need one 5 string tuned down a half step and a 4 string tuned Drop D. The combo of those cover everything down to Bb. However, if you're playing low Bb live, no one can hear you. God help you if one of your basses is passive and the other is active.
Find a new band...
You CAN play ROTM and Tool and Creed on a 5 string, but you'll really want that drop D tuning on the 4 string. But here's your issue, if the problem solving tuning above is your way forward, you're going to be actually tuning down to C# and you'll be getting kinda floppy. But...your singer is now used to the 1/2 step down which is understandable. He's always been a taker. But his problems are your opportunities.
You'll need one 5 string tuned down a half step and a 4 string tuned Drop D. The combo of those cover everything down to Bb. However, if you're playing low Bb live, no one can hear you. God help you if one of your basses is passive and the other is active.
Find a new band...