Biggest Sound in the Littliest Box

"Big speaker sound?" Hummmm...that one might have to be quantified more for me. If you're refering to a bigger speaker as sounding more"full, "weighty", and "authoritive" in the bass reigion, then that's one thing. I've certainly heard speakers that offer a MUCH BIGGER SOUND(potentially, considering good room acoustics and set up) in terms of soundstage, than others!(refering to pair) Infact My older SF electa Amators and MInima Amators produced a HUGE SOUNDSTAGE in a proper set up! The sound was "bigger" than what you'd get from most any speaker, regardless of size! That Dynaudio Essotar 330 tweeter produced as much detail and life as most anything I've heard!
So, when you say "big" I'm just not sure what's being infered.
Also, I'd submit that tiny speakers, even if they have better than average bass response(if that's what's being refered to), will still need a powered sub somewhere to pull off the "full range thing" properly...otherwise you've got a limited aplication monitor...if that's what you're after.(which for some applications is ok) That said, if you're after a monitor that produces a big sound(probably refering to size of soundstage) within its respective frequency response, and you add a well set up woofer somewhere,you should have some overall "big sound"!
Of the "tiny" monitors out there, I have a hard time finding "a pair" that produce a HUGE SOUNDSTAGE personally. You might consider doing multiple minimonitors in a multi channel set up for that.

I would bet that if someone could build a smalle speaker cabinet to house a dynaudio Essotar 330 tweeter and high end 4" driver, and integrate a proper crossover(challenging?), you'd have HUGE SOUNDING MONITORS when mated with some well set up good bass woofer(s)!!!

Quad 12L...and real bass to boot...I cant get that excited about any monitor that barely does 60 hz...go full range...or dont go at all...

Role Audio makes an amazing sounding very small speaker. It is about 6"x6"x6". The sound is very real and natural. It has a single driver and no cross over. Very interesting loudspeaker.

Third for the De Capos... if you're looking for a small speaker with impressive bass response and that can handle a good deal of power. 300 Watts is overkill, obviously, (I'm assuming your Classes is 300 watts) but the De Capo can play loud and full. See my system--I'm using 150 watts, and the speaker doesn't even blink.
However, if you want deep bass repsonse (solid response under 40 hz), no small speaker will do it. Look into subwoofers. If space is an issue, take a look at the Virtual Bass Technologies subwoofers (http://www.subwoofersolutions.com/). They are remarkably small. There is a review on Absolute Sound's AVGuide.com site.