Can you explain more? I thought a sub just outputs lower frequencies/bass. If I want to blast let's say Bach's
Chaconne (solo violin), how will a sub help here?
Also, what size of speakers (or other properties) do you mean by
big speakers?
[Re when I said "To vastly oversimplify things,
big sound needs big speakers (
or, to cheat in some sense, smaller speakers with a big subwoofer"]
Well let me repeat the "To vastly oversimplify things" part ha ha. If you want to blast solo violin, or piccolo, or solo female voice, or home recordings of bats, the sub will help not a bit, you are correct. When I think of the spectrum of classical music I think of pieces that generally DO have significant bass content. Many big towers like the Focal 936 (4' tall) are ported down around 40 Hz, which would suffice for an awful lot of that, excepting long organ pipes, the largest kodo drums, and such.
"Big" is a relative term. There is a thing called "Hofmann's Iron Law" (the "H" in KLH) stating that speakers are a tradeoff between how big they are, how low the bass goes, and how power hungry they are. IIRC he said you could have any two out of the three. I'd further posit "Head_Unit's Limitations" which is that those tradeoffs are not
all achievable-you can't have a speaker* flat down to 10 Hz the size of a coffee cup because you literally would not be able to physically build a suitable driver. Even if you could, the small size would not be able to output enough volume at that frequency for you to actually hear it. Sound pressure level comes from moving air, and every time you halve the frequency the cone must move 4X as much to maintain the same sound pressure.
How then do some people claim that their favorite speaker with only a 5" woofer has great low bass? Well, if it's ported low that can help the output, as the port reinforces the woofer output. Generally however a small speaker cannot be ported very low effectively. Gain from the room can help some. There's also a psychoacoustic thing where if you hear the overtones, your brain "fills in" the fundamental note (this is exploited in DSP for some of the very small Bluetooth speakers). And if the midrange and treble sound great, some folks are willing to get used to a lighter weight bass presentation.
To me, to be "big enough" for low loud bass means an 8" woofer or twin 6.5"; the Wharfedale Linton Heritage is an example of what I'd consider about the smallest thing I'd personally consider a true full-range speaker for regular music. For those ultra-low musics and movie effects, to me "real" subwoofers have a 12" cone and 400+ real watts.
*headphones are a different matter, since the speakers is right next to your ear and often sealed to it.
Oh, and all the above is without equalization. EQ boost of low frequencies is essentially a way of meeting Hofmann by applying more power at some frequencies.