My response won't help you, but I have considered writing the very same thread! I've owned two sets of TC-50's for almost 20 yrs. now... always wondered about the LS3/5a's but have never heard them...
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I've owned both in the past, the Spicas never able to dislodge the LS3/5a, however. The Spica appealed to many people because of its greater dynamic range and better bass extension over the BBC monitor. But the Spica could not "disappear" like the smaller LS3/5a. The box is always present and audible. Additionally, the Spica is no match for the 3/5a midrange beauty and clarity, and its woofer and tweeter are dynamically disjointed in woofer/tweeter transient behavior. The Spica woofer sounds slow compared with the 3/5a B110 Kef driver. Last, if I remember correctly, the Spica is nominally rated at 4 ohms, dipping to a low of 3.7 ohms. That's not a load at which most solid state amps sound their best, even if they operate fine. On the other hand, the LS3/5a is 15 ohms or 11 ohms, which extracts better sonics at lower power from most solid state amps.
The Spica is a good speaker, and was especially so in its day. It has a persistently warm vocal balance that some people like. But for me, it was no contest years ago. The LS3/5a was simply more objective and expressive than the Spica, and it's difficult to find both points of advantage in one speaker, let along packaged in a camera that small.
I have owned spica angelus and tried Ls3/5s for an extended weekend demo and the imaging of the ls3/5 is nowhere near that of the spicas.
I couldn't believe it at first given that the ls3/5s are considerably more expensive, but after 2-3 days when the spicas were replaced the pinpoint imaging (totally absent on the ls3/5s) returned.
I eventually changed my spica angelus to Green Mountain Audio Europas because the spicas were too difficult to move. The Europas are slightly more forward than the spicas but better in all respects, including imaging.
Comparing Green Mountain Europas and LS3/5s it's not even close. The LS3/5 is a nice speaker ... unoffensive, but it is an ancient design. The only way in which the LS3/5 could be preferred is that it is less revealing of upstream equipment, and produces more of a "romantic" sound than the Europas.
With respect to sound of solid state amps and speaker impedance, I am reporting results, not "common wisdom." Common wisdom tells us a lot of things that just aren't true. Some speaker designers are formulating to 6 ohm nominal loads because "most solid state power amplifiers are designed to that optimum." But I've yet to hear a solid state amp that sounds better at 4 or 6 ohms than it does at 12 or 16.
Anyway, I don't care about common wisdom. I'm interested in the resulting sound. The poorer top end quality of the Spica TC50 compared to an LS3/5a on most solid state amps is consistent with the deleterious effects qualitatively that can be heard when, for instance, a double set of 8 ohm speakers are wired in parallel and series for comparison.
Irrespective of the Spica's strengths, it sounds tonally incorrect to me compared to the LS3/5a, and quite apart from that I think its qualitative character -- even if the design decisions driving tonality were left intact -- would prove improved on most amplifiers if its impedance were 8, 12, or 16 ohms. "Most amplifiers," of course, implying solid state by virtue of their incidence in the amp population. In 30 years I have never heard a solid state amp sound its best into 4 ohms or less, though I certainly assume there must be exceptions.
The LS3/5 might beat the Spica on tonal accuracy ... I'll admit that. But the margin of victory would be quite small IMO.
On the other hand when it comes to imaging and casting a wide deep soundstage there is no competition. The LS3/5 is not even close to the Spica. It is this that makes the Spica such a legendary speaker.
As to the issue of impedance I would think it would depend on the amplifier, its rail voltages and its ability to deliver current and double its power into 4ohms. Some amps can't deliver current (older Quads) and sound much better into high impedances. Others (Cyrus, Densen to name a couple) deliver current in spades and seem much less fussy about impedance.
Yes, I do recall that at the time the Spica was revered for its imaging. I could never get past its serious tonal colorations to choose the speaker over an LS3/5a. To me, in that respect the margin is not small at all. I did not (do not) consider the Spica TC50 to be tonally credible, but it is euphonic so I understand why a lot of people like it. Now if one prioritizes imaging over tonality, then there is more of a case, although while I agree the Spica beat the LS3/5a in image expanse and depth, I did not hear it as having the precision and stability of image that the BBC monitor had. Someone else might hear that aspect differently, however.
The impedance issue isn't a current issue, with respect to the consistent poor performance I hear from solid state designs in low impedances. Even a Cyrus doesn't make it, though all other aspects of the sound produced indicate ample current delivery. Current abundance or current started, I consistently hear solid state amps into low impedances take on more grain, less fluidity, less dimension. Sometimes the difference is subtle. But once you get sensitive to it, even small differences argue for the sound at higher impedances. I haven't interpreted this as an issue of current.
In the bass, no question the Spica is more extended. Qualitatively, it still has a euphonic rise, and the transient behavior of the woofer isn't particularly fast. It sounds a little lazy to me, but again is euphonic.
Certainly the LS3/5a, and the KEF drivers it was based on, are dated but no less classic. At the time of their debut, it was as close as you could get to a Quad-like midrange in a dynamic speaker, and it's stiff little box didn't make itself heard like larger models. All things considered, the Spendor update, the S3/5 and S3/5se are superior and work with a wider variety of amps. In the Spica style of emulating small monitor performance in a somewhat larger form factor, the Silverline SR-16 is tonally more accurate and the woofer speed is a better match for the tweeter characteristics, IMO.
The biggest problem with the LS3/5a in a modern context is its poor efficiency and limited power handling. You have to put reasonable limits on the size room you expect them to drive, to avoid frequent sense of compression.
As for LS3/5a bass, it is euphonically manipulated, by design, with its mid-bass hump for illusion. But late in the speaker's tenure in my systems, I learned its bass was better than I believed when I put them on a pair of Osiris 24" sand & lead filled stands. It was the single most dramatic improvement I'd ever made to an LS3/5a system, considerably outstripping the impact of any one change in the signal chain.
Joe-k: Yep - I've owned both of these "monitor" type speakers and while I was happy with the LS3/5a for many years I don't think I'd go back. Specs wise BOTH are going downhill past 17K so there's not that much difference there. Perhaps a nod to the 3/5a's. Otherwise, the SPICA has it all over the Englander. More extended Bass, greater dynamics - even better imaging.
The LS3/5a was just not MADE for domestic consumption - it's too easy to over drive and it's difficult to match with a sub. it works best as it always has - as a monitor set up a ear level and toed-in towards the listener at near ear height and in close proximity.
Of course, you're on your own when it comes to support for the SPICAs; the LS3/5a has a very active usegroup with great worldwide communication. Also - many, many more of those famous "shoeboxes" are avail, lots having been produced by the Beeb's licensees as studio monitors down the years.