Do we really need anything more than 8" woofer and 1" tweeter for medium size room?

With the right electronics good 8" can go down very low and relatively clean. And if that's not enough, well, two 10" subs would do it. 
Depends on the music. For lots of commercial "Pop" music, the lowest note in a recording may well be the open E string of a standard 4-string Bass (electric or standup), which produces a 42Hz tone. For Classical organ works recorded in a Church, that's an entirely different matter; the lowest note produced by the far left foot pedal of the largest Pipe Organs is at 16Hz!
The Spendor SP1 is still one of the best speakers I know. Have a pair and rotate it with bigger and more expensive speakers. Will not play insanely loud or get down to 30hz. Use a REL if you want low bass. If I had to take one speaker with me it would be the one.

My speakers have two 6" TL drivers per side, and a ribbon, and I augment them with an EQ'd 15" sub in my 24' x 20' room--but that's a personal choice. I listen mostly to rock, pop, and old-school R&B and country, and my combination of speakers/sub delivers a decent amount of impact in my treated room. But I think the choice depends a lot on room size and listening preferences.
The big unaddressed question from the OP is "What constitutes 'relatively clean'".   At 85db in a mid-sized room, most 8" woofers are distorting horribly.  The good news is that the ear isn't terribly sensitive at such low frequencies, so you don't necessarily hear it ...... Until you switch in lower distortion (read larger) drivers to handle the bass.

To get a visual sense of this, visit the subwoofer tests section at  You'll see that a purpose built 8" subwoofer driver (like the Velodyne SPL8) produces vastly more distortion tha a good 12" sub (say Rythmik) at the same frequency/spl.  If you think that tests aren't indicative, I'd suggest you actually try it in you own home.  I did, switching out the little Velos for the Rythmik 12s. Instantly evident improvement.

Bigger Drivers tend to work less harder. Small Drivers work harder.

The Room is very important, smaller drivers disappear easily with not much effort put into the Room acoustics. Larger drivers can disappear just as well but you need to work on the room further.