I use an old GK 400RB too, it's a fantastic little amp.
One thing to know though, is that the 'Direct' out from these amps is unbuffered & comes straight off the power amp.
When you're playing live, I'd imagine you're running that amp fairly hard, which results in that big GK signature growl, what I believe Robert Gallien refers to "hitting the rails" tone. Maybe in the studio, you're not running the amp as high as you would do when gigging?
I'd go with Vail's suggestion, try disconnecting the speaker & run it pretty flat out, see if you're acheiving that tone you're after. Be wary though, these amps get hot!