Basic question about sensitivity

Any significant upgrade with modern speaker can turn out a downgrade so take that into consideration. It was the best monitor they've ever produced. They're easy to drive with tube amps, because of their stable impedance. 

Thank you for responding @czarivey  , to be totally honest, I do not have a great understanding about this hobby, but I suppose in one respect, I am like most everybody else, and always wondering what the weak link is and what it would sound like if I upgraded past it.  When I bought those B&Ws way back in nineteen-ninety-something, I was so happy with them that I didn't even think about them as a weak link and I kept on upgrading the electtronics in front of them, and lately I was thinking that I had the front end to speaker ratio skewed.  And that is what prompted the recent "windo shopping" I have been doing.

@OP There is no problem using a 4 ohm speaker with your V12 as long as the speaker is fairly close to it's nominal impedance. But you probably want something in the region 87dB. But if you are sill happy with your 805s, I wouldn't be rushing out to change them.

Any significant upgrade with modern speaker can turn out a downgrade so take that into consideration. It was the best monitor they've ever produced. They're easy to drive with tube amps, because of their stable impedance.

But if you are sill happy with your 805s, I wouldn't be rushing out to change them.

@czarivey  and @yoyoyaya  thanks, that might be the best advice for me to live by for the time being. 

AOP - you're welcome.

As many have posted, the nominal impedance varies with frequency.  Getting lower efficiency speakers to make a sound with a few watts isn't that difficult, but getting the most out of them with insufficient power is.  Nearly all sensitivity ratings are made at 1000 Hz, so it's just a tiny slice of the spectrum we're attempting to amplify.  For example, if the speaker is 3 db less sensitive at 100 Hz, it will take 2x as much power at that frequency to produce the same volume as the 1000 Hz tone.  And if the impedance drops to 4 ohms, for example, at 100 Hz, then the current demands will be higher for 100 Hz than 1000 Hz.  I used to have Thiel CS3.5.  They were the first speaker I owned that really needed power.  They were excellent speakers for weeding out the amp collection.  Then I discovered Magnepans, which had a hard to drive, must have power reputation.  Yet they weren't nearly as difficult as the Thiels.  None of my tube amps could drive the Thiels well, while the Maggies love tubes.  Moral of the story, don't read too much into the specs.  They're a single data point.  Music is a nearly infinite amalgam of data points when you consider the frequencies and amplitudes at play.  Listen to speaker candidates with the amp(s) you have, or better yet, choose the speakers you like best, then find the amp that pairs the best with them.