If you like the sound of the LB, get a cab that is loud enough with it, probably two cabs.
Valve type and number isn't the determining factor for wattage, that depends on what they are doing in the amp, circuit style and suchlike. Ditto with the tone, putting the wrong valves in an amp gives the tone of an amp with the wrong valves in it, which you may like, but the tone isn't tied to the valves, same valves in an amp with a different circuit will sound more different.
You can put 6550 in a KT88 amp but putting KT88 into a 6550 amp can be a bad idea as KT88 draw more current, however a lot of KT88 are actually 6550 in a KT88 shaped bottle.
Tons of vintage amps suitable for bass with EL34. What cab do you have?