they're 100% identical, btw. it's just that the block letter heads were shipped with better stock tubes, which leads people to think that they are better sounding per se.
There's absolutely no difference between the block letter and sig 5150 heads other than what tubes they may have initially been issued with from the factory! The internal schematics are identical!
Many notable techs over at Harmony Central stress this point, including Jerry P who specialises in 5150's and modifications for them! That man knows those amps like the back of his hand!
Regardless, people do still think they can hear differences
I got a 5150 back in 1994 when I was a senior in high school. I caught so much shit for buying it from all my other friends that played in bands. They thought the amp was a joke.
Maybe because it was 1994 and the amp was Van Halen's sig model?
I mean, he's no grunge/alternative hero of any sort, in fact it was popular to "hate" such players back then...