LS-50 Speakers (Passive)

The Elac UB5 is a very good speaker for the money, but it IMO it doesn't sound anything like the LS50 passive.  The imaging and dispersion pattern of the dual concentric UniQ driver on the LS50 in entirely different than the UB5, which sounds more "conventional" (not necessarily in a bad way).

While the LS50 is not my personal choice it is a superior and very distinctive speaker.  The UB5 is distinctive mainly for it's performance at it's price.  Again, IMO.

I would only get the LS50’s if you plan to also use a subwoofer (or two). They are great speakers, but they only get to 79Hz +/-3dB (ground plane, so some in room boost).

The Philharmonic “New Philharmonitor” would be my pick if you won’t be using a subwoofer. They are built to order though.

The UB5 (only addressing it as others are recommending it) has a midrange boost, and it’s imaging is pretty good, except in the 3kHz-7kHz region.

The HSU CCB-8 is very similar to the LS50, but a decent bit cheaper, and has a good amount of bass, so subwoofers aren’t a necessity.  
 
 The SVS Ultra bookshelves are also very good. However, I feel they are a bit overpriced. Via their outlet store, you can get the Piano Black finish, one being undamaged for $50 off and one with faint scratches for $100 off, they post pictures of the scratches, so that would be $850 total instead of $1000.

I have a pair of UB5s, as well as a pair of ~1978 Chartwell LS3/5As. I should compare them some time.

40 year old caps are probably suspect

crossover to my sub naturally at the point where the mains can't no longer output, and keep the use of the sub to a minimum.

crossing over an octave above the main 'lower' frequency @ 6db / octave with a passive XOver and setting sub[s] to 18db / octave and 180° with the sub[s] in the main plane yields better results than trying to get bottom out of a small driver / box. The phase is correct, the mains are not stressed and less power is required for the mains. Main and sub should be same type, i.e. sealed or vented.

C = 1 / ( 2 * π * f * R ) where f is desired frequency, R is amplifier input in Ω.
C is in farads, so multiply by 10^12 FOR pF

I have a pair of UB5s, as well as a pair of ~1978 Chartwell LS3/5As. I should compare them some time.

40 year old caps are probably suspect

Original LS3/5As can be worth $1,500 to $2,500 a pair. Modified ones ain’t worth sh*t :-)

The LS50s (recently manufactured examples) can compete with many ~$5K speakers IME. However, getting them to do that requires a high quality amp and source. If you pair them with a budget amp, say something in the <$1k range, as one likely would with $1500 speakers, you won’t realize their full potential, especially concerning their bass. This is why some find budget speakers like the Elacs to be nearly as good. It’s because they have system bottlenecks elsewhere.

It’s better to think of the passive LS50s as a $3500 speakers, because they simply require high quality gear. I run a pair with a budget amp in a third system. They sound ok for that application, but having had them paired with $2.5K amps in my other rooms, I know they’re capable of far greater performance. I wouldn’t read too much into comparisons with lower KEF models. Stereophile recently rated the Q350s as "class C" speakers while the LS50s remain "class A." I’ve owned and auditioned a fair number of their class C rated soeakers. The LS50s are in a whole different ballpark when properly powered. The wireless version is a better value proposition unless you already own a high performance amp.