The RBI might sound great to this guy, but (1) doesn't have a place to stick a tube nor (2) is a kit.
I have a Blue Tube pedal that can get pretty grindy, depending on the tube in it. It's a fairly simple circuit (Fender tone stack EQ) that you can find vero board layouts for online. I've heard good things about the Mojotone 50W bass tube amp kit, as well. Solid state kit stuff doesn't get much love. I think the common DIY bass amp approach is to roll your own tube preamp and just buy a big power amp to run it through.