All depends on your tastes. The 5150/6505 is just a more aggressive SLO with enough gain to me. It needs a boost to do it's thing best, in my opinion. I'd honestly rather own my 6505 over a SLO, too; the 6505 has that extra grind and fatness that is characteristic to a lot of modern metal.
However, the Natas doesn't need anything, aside from two cabinets when the master is close to 2 O' clock The reamps I'll be doing in the near future with that thing are going to blow minds