using different size woofers

Can't hurt to try and find out. Phasing might be an issue (cancellation) but considering some home speakers do something similar to this it may work ok. The thing is the speakers are crossed over to minimize "overlap" and avoid cancellation..the frequencies an 8'', 10'' and 12'' sub can reproduce are pretty close so using cross overs is probably a waste. If you try this make sure they do not share the same air space..I could see that causing the smaller subs of over excurt.

Lets us know what happens if you do this..personally I find one 10'' enough to cover all the sub freqs. needed to enjoy music.

What if you used an ultra high end 8''...a medium quality 10''...and an entry level 12''??   Would they all sound the same??  Jus kidding.  lol...but absolutly a factor if the goal in doing this was to "cover all the frequencies". Logic tells me there would be no point in doing this but experience tells me it would work..well somewhat...in other words there will be an increase in sub bass output at the frequencies the subs reproduce working together.

But then again if a basshead loves shaking the neighborhood @ 30hz I don't even think an 8'' is a consideration

Yes, that application makes some sense. When I am designing a system utilizing 10'' or 12'' subs the lower spectrum is covered. In that case if I was considering adding an 8'' to this system I would bandpass it and use it as a midbass and place it up front if possible such as in a custom enclosure cut into the floor or in the doors if possible.

I don't think there would be much advantage to install an 8'' sub in the rear if you already have 10s or 12s there as it would pull the sound stage back, not the best for sq.

I agree.  And if the 8" were combined with the larger sub under the same crossover, it wouldn't make any difference that it were a smaller cone area driver.  Except that the sound quality would most likely suffer because of the mishmash of different driver sizes being used to produce one part of the total sound spectrum.  It's like, if you usually don't see such a sub arrangement done, there's good reasons why you don't.

Questions like your friend posed, and as it reads in your post, seem to normally come about because of the novice desire to "have the best of both worlds", or "reinvent the wheel".  They are burdened by their lack of knowledge and experience on the subject.  Like the people who can't comprehend the distinction between SQ and SPL, and so carry on with their own "SQL" terminology.  Or the beginner who strings 23 flea-market drivers around in his beater and assumes that gives him the distinction of being a newly-crowned King of car audio and recognized audiophile expert.

You hit the nail on the head, I wish to add only one thing. A lot of people are going by myth that is created by "experts" in this industry. As the new year has followed a lot of sales reps representing different brands are about trying to push/market there new line ups for the year. It is amazing the amount of b.s. some of them spread in which rolls down hill to the sales/install staff and eventually the customer. stevdart, dyohn, hemp, forbidden etc....... you cats have enough knowledge to baffle many (inc. me :) and your information is valuable  for all especially  beginners please continue to share....

And yes a rep as early as today commented on the vast "SQL" potential of their new subwoofers for '07, I corrected him but he was mecp certified for product knowledge...........................