Help! 4th tube rig to give me problems

[DownLowDelta]

I'm actually asking this on behalf of a friend...

He has had a bad string of luck with blowing up heads and/or cabs (at reasonable volumes), so I want to help figure this out once and for all.

He plays an active Fender Jazz Bass through an Ibanez Tube Screamer into a Mesa Bass 400+ tube head and (I believe) an Ampeg 600 watt cab. 1/2 step down and drop D for tuning.

I'm a guitar player and my instinct tells me it's the Tube Screamer. Switch to bass overdrive?

OR... on the Mesa... he has the preamp gain pretty high and power amp gain fairly low. Is it just too much gain before the power amp (esp. since it's an active bass)??? Less pre-gain and more poweramp "soak" maybe?

I'm kinda new to the all-tube thing for guitar too, so I wanted to be careful about how much I'm boosting or overdriving the signal before it hits the power amp.

ANY feedback, suggestions, and advice is extremely appreciated.

[Rick Auricchio]

Without knowing exactly what "blowing up" means, there is no way to offer a diagnosis.

Have you considered power problems? That would be common to all amps he uses.

[Selta]

What do you mean by "Blowing up heads and/or cabs". We need more detail on what actually broke before we can even begin to guess or give advice.
My first thing would be to make sure that the impedance on the cab matches the head. Does the 400+ have an impedance switch? Is it in the correct position for the cab?

Edit: I very highly doubt it's the Tube Screamer, the active bass or the EQ settings on the 400+.