I can't reco a specific bulb 'cause I've not tried them, jus' spent time staring at the selection and wonderin'... then I got Norm's lights heh - but I will be needin' one (three actually) here pretty soon as I've got a 6v gen hub & lights (different bike)... but no bulbs as of yet. So ifn ya find something nice - remember to post it
While fiddlin' around with different bulbs is a worthy cause I still think it's prolly the reflector/lens style bein' more a "flood" than a "spot" that's causing it to seem dim... If it were my ride and a different bulb didn't help - I'd just get a second battery-operated light for those times ya need a high-beam and keep that white wire light on all the time as a safety feature, so ppl can see
you *shrug*