For getting strong, clear tone in the upper clarion, first make sure your horn doesn't have any leaks, bent keys, loose screws or the like. A lot of times this can solve problems you didn't know you had!
The keys to getting a good sound on the bass clarinet are a clarinet embouchure with the tongue arched high in the mouth (think "heeeee") and good, strong air support. A lot of sax players don't arch their tongues enough when they play bass clarinet, preferring instead to use a tenor sax style embouchure, throat and tongue position. Consequently, a lot (I'm not going to say all) of these players have tones that are buzzy, spread, unfocused and lacking in a lot of "core" sound.
A good mouthpiece can really help to facilitate good tone through the range of the instrument, but before you buy a really nice mouthpiece, I would recommend finding something that works (Yamaha, Hite, Bundy, used Vandoren) while you learn to control this beast. Then, when you're good enough to know a good mouthpiece when you play one, you'll be able to make an educated, informed decision and get the best one for you.
Disclaimer: All of this is what works and has worked for me, and may not necessarily be what works for you.