I have an early 70's 1959 SL and to be honest, it's one of the best sounding amps I have ever heard. You definately have to crank it to get the best out of it.
Having heard a 1959 SLP from the late 60's (I know someone with two, although he won't let me have one...), the SL sounds very similiar but has a tad more gain IMHO. The modern reissues do not even get close to my ears.
Depends on what your tonal bench marks are in terms as to what is the holy grail. I have a fair few amps and each one exhibits a characteristic that others cannot attain.
Do you need the volume? If you are playing pubs or small to medium venues I would strongly recommend a Matamp King Street head and cab. It's in the early Marshall ballpark but sonically far more versatile, is built like a tank and the attention to detail something else. I should know... I have one!
It's conservatively rated at 15 (clean) watts, but when dimed it's probably nearer 30. However... I used mine at band practice last week and it was easily heard over a 100 watt Engl head... the tone was amazing as well.