I've used all the common bass amps over the years. V4, SVT and a whole boat load of GK. I'm also a long time fan of the Avalon U5 on bass. Doing FOH for a band I respect last summer, I took the opportunity to not just throw down a DI for the bass but used the on board in his Mesa D-800. His amp was sitting on top of his cabinet, not buried in a rack so I could see what I was doing. Set to Pre EQ. Wow... So i bought and put 10 pr so gigs on a Subway D-800 and I now own a Mesa D-800+ as my personal amp. The DI is as good as my Avalon.
Later had an on line chat with the designer. He turns out to be a prosound guy as well. as he puts it, a DI designed by a prosound guy, for prosound guys...
If your bass player is thinking about changing things up, the Subway D-800 and 800+ should be on his or her list. Sparkly clean to very warm and slightly loose is all on tap in an easy to suss front end.