In the old days, in the Hi-Fi world, they used to say that you should split your budget, spending 50% on the speakers and 50% on the rest.
Having only played boutique-ish (Dynacord, MM, Mesa, Aguilar, Epifani, Bergs and Barefaced) cabs since the mid 80's I'd say that a quality cab will have the greatest effect on improving the sound of an average amp, bass or player. It's the one component in the signal chain that can do this. If you don't have a good cab then everything else is fighting a loosing battle.