For pure volume very few watts are needed for the big horns (K-horn/La Scala). Like many posters I have used some pretty low watt amps over the years and volume was never an issue.
I have found however I strongly prefer a robust SS amp on the bass bin. The lower watt amps loose bass definition/articulation compared to a good powerful SS amp. Conversely, a smaller amp with a few high quality watts is what sounds best on the mids.
For me the perfect combo is a nice Crown class D amp on the bass bins of my K-horns, with a really clear lower watt amp, currently a Topping PA5, on the upper horns (in my case K402s).
Also, time alligning makes a HUGE improvement in bass definition as well.
Everyone hears a bit differently so...YMMV.