I used LS3/5a speakers off and on from 1976 until 2002. It is, like a Quad ESL57, its own thing, not precisely duplicated by anything else. It is amp sensitive and definitely won't sound thin if driven properly. They need some oomph yet have limited power handling. The original 15 ohm design happens to make many solid state amps sound much better than they have any right to, and an unbloaty tube amp can be nearly heaven, within the limits of the speaker. Also, what you put them on can have a dramatic effect on their performance, especially in bass definition. The best sound by far I ever got out of the LS3/5a was when placing them on Osiris 24" stands, loaded with sand and shot. They're not made anymore but you can find them on Audiogon and eBay occasionally.
The LS3/5a is a speaker that benefits fully from amplification considerably more expensive than the speakers themselves. The VAC Avatar and Super Avatar do a spectacular job with them. But for much less, the Prima Luna integrateds do well. The EL34 gives you finesse and a softer sound, the KT88 version tightens and speeds up bass attack, and projects more energy.
No question there is a certain euphonic datedness to the sound. The crossover is complex, and clearly contributes to the speaker's relative inefficiency, 82.5db/w/m if I recall. The KEF B110 mid/bass driver has a bextrene cone, materially advanced for the early 1970s. Compared to well-made paper or modern composits, however, it's a little slow and warm sounding. The speaker's colorations are all euphonic and carefully chosen.
The current Quad Classic II and KT88 monoblocks are terrific with LS3/5a too. Also, if the spend doesn't frighten you, an 845 SET at 25w or so is perfect and can make magic. But there are many perfectly affordable push-pull tube amps and moderate SS units on the used market.
The Spendor S3/5 and S3/5se speakers are spiritual successors to the LS3/5a, designed in the late 1990s after KEF stopped production of the B110 and T27 drivers on which LS3/5a production depended. The newer Spendors have a decidedly more neutral, accurate sound with appreciably better bass and treble extension and better dynamics. Less amp sensitive too, and you get a small 1.5db bump in efficiency. I recommend them and they can cost less than a mint pair of used LS3/5a. By comparision the S3/5 and the se version are objectively better speakers than the LS3/5a, but in the very core midrange where many of one's perceptions of musical magic are formed, the LS3/5a has an indescribable edge that makes you wonder whether it's better to live with the flaws. Every former Quad ESL57 owner understands this perfectly. I've been down both roads for extended periods of time.
Phil
