Good question! I don't think I've seen or heard of any Sports bars in Madrid apart from billiard-bars but I think that's as close as they get.
NBA games are easier to come by than baseball or American football games in Spanish bars but I doubt there are any which promote the games as "NBA SHOWN HERE!" You might walk into one and catch it by chance or, if they're not already showing a "soccer" game, you could ask them to find the game on satellite. Plus, the bars which DO stay open late are the night club-type bars and not the kind which typically show sports on the TV above the bar. The latter kind of bar usually closes relatively early. Even the Irish Pubs which show a lot of soccer games are usually closed, I think, by 2am.
As for Boxing, welll...... I seriously doubt (nearly 100%) that you'll find this in a bar for the following reasons:
1) this would be a Pay-Per-View game in the USA and probably is the same here in Spain and no bar would pay to televise a game in their bar (unless it was a world championship game, maybe, or a Real Madrid versus Atlético de Madrid match)
2) this match likely starts at, what, 10pm EASTERN USA time which is 4am SPAIN time soooo... nothing would be open anyway
Your best bet for this is to find someone with satellite like Canal+ or Digital+ and offer to A) pay them to pay-per-view the fight and B) ask them if they wouldn't mind if you stayed in their house until (potentially) 5:30am on the sofa and drank all their beer and ate all their potato chips. hehehe..
If you find such a person let me know. HEY, *I* could do that!!! But I don't think I want to stay up all night either.
If you happen across a
Hooters bar (or similar - for the sports) open until 6am please do let me know! I was fortunate to finally find - with the help of a message board member - last year's Ohio State University Vs. University of Michigan football game at an Irish Pub nowhere near where I live here in Madrid.
Saludos, MadridMan