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

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 htshack.com.  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.

FWIW 

Tim,

My statement re distortion at 85db wasn't loaded, I cited the source - htshack.com.  Go look for yourself.

However, I neglected to specify one condition and should clarify a second.  

I was focused on low bass, but didn't mention the frequency at which a purpose built 8" subwoofer distorts "horribly".  So, go check distortion at let's say, 55 hz ( or lower). I'll choose 55 hz because it's typically close to the fundamental of a kick drum (no pipe organ needed to demonstrate the shortcomings of an 8" driver).  As you go lower in frequency, distortion increases dramatically.

I will note that the tests are "quasi anechoic, measured at one meter".  Some could argue that this will produce results closer to a large room than a medium sized room.  I could make a case either way, but I'll leave that to everyone here to decide on their own.

Unfortunately, htshack.com seems to have removed most of its subwoofer test database.  By memory, I wanna say that the 8" subwoofers typically produced about 4 or 5% THD at 50hz at 85 DB (in a quasi-aechoic set up that effectively removes room nodes).  At 95 DB, that number more than tripled.  At lower frequencies, performance is much worse.  

Subs with larger drivers fared much better.  I switched from a pair of 8" Velodyne SPL(?) subs to a pair of Rythmik 12" subs and the A-B comparison was pretty compelling to me.  The ear is pretty insensitive to THD at these frequencies, but the sound "cleans up" in a very nice way with bigger drivers IME.

I'm pretty sure that there's a database out there somewhere with more detailed info, but I couldn't find it on a first search and I'm too lazy to do another.

Don,

Most two-way, 8" woofer models that I’ve seen cross at 2200hz or higher, well above the middle of the mid-range. OTOH, lots of three way speakers cross to the woofer between 300 and 400 hz which is lower than middle of the mids, but closer. Maybe you’re just looking at a different set of examples than I did, but if you want your crossover out of the mids, I’d think you’d have a better shot choosing a random two way 8" over a random three-way model.

OTOH, lots of people think a x-over in that lowest part of the treble/presence region is worse than one in the midrange. I’d say that it’s Personal taste on that one.