I play an older GK400RB into an LDS 1x15. In fact, I have two identical setups. I use one cab for regular club gigs; two cabs for louder gigs; both cabs for festivals I book, using one head as the main and keeping one for backup just in case. That just in case has never happened.
As much as I keep trying other heads and amps, searching for 'the' tone, there's something about the 400RB that just suits the jump blues stuff I play. They're simple to use and built like a tank. I've never -- in 18 years playing bass -- had a problem with a GK unit, and I've had 3 400RBs, a 1001, and two MB150s combos.
I think they're plenty loud on their own, and if the gig calls for more, there is (or should be) PA support to run through.
That said, I did have (and got rid of for financial reasons) an all-tube Morgan MP200 that blew me away. Of course, it cost 7 times as much as a 400RB, but man did it sound sweet. I'll get another one one day, but I'll ALWAYS have a 400RB ... just in case.
"I've said that playing the blues is like having to be black twice. Stevie Ray Vaughan missed on both counts, but I never noticed." -- B. B. King