More speaker mass more air, you would need 5 18'' or 6 15" to move as much air as a single 8x10. More speaker surface will sound fuller and thicker. I use to use 2 8x10 back in the day (80's) yeah I am not 20 years old anymore. although I am pretty sure they had a lower wattage rating, they were old and from the 70's.They were sealed with no horns and was a awesome hard rock set up. I powered them with a Peavey Mega bass ss amp I believe was 500 watts and it was screaming loud and super punchy and was plenty enough with are guitar player with 2 full stacks. It don't matter if you have a 4ohm 15'' speaker cab that can handle a 1000 watts rms it will not be as loud as a 4ohm 800 watt rms 8x10 powered by a say 500 watt head @4ohms. Personally I don't want to lug that around anymore. All being said:My vote, Hartke vlx 8x10 paper cones sound good and are cheaper then Ampeg, I played out of a demo and it was on sale for 400.00, I almost bought it myself.
that is quite wrong reguarding the speaker surface area. you must have been thinking diameters when calculating, which will give you a very different answer.
i calculated the speaker area of 8
http://www.eminence.com/guitar_speaker_detail.asp?model=BASSLITECA2010&speaker_size=10&SUB_CAT_ID=5 which are 10" speakers. and then found that you could get the same surface area with 3.4
http://www.eminence.com/guitar_speaker_detail.asp?model=BASSLITEC2515&speaker_size=15&SUB_CAT_ID=5 15" speakers. granted not all speakers of the same size will have the same surface area, but they will be close enough. these were just random choices.
another thing to consider is xmax. larger speakers will tend to have a higher xmax (maximum excursion). using the same examples the 10" had 3.5mm xmax, and the 15" had 4.5mm.
doing a very rough calculation (multiplying the xmax by the surface area of the cone) we should be able to get an idea of the air that the speakers are pushing.
360.7cm^2 x .35cm = 126mL per 10" speaker
856 cm^2 x 0.45cm = 385 mL per 15" speaker
so 8 of the 10" speakers is 1008ml of air being pushed
that's the same as 2.6 15" speakers. that is nowhere near 6 15" speakers.
these things effect the low end mainly, and often the mid's effect the percieved volume much more than the low end, but mids are quite easy to get, its the low end that is difficult to dial in when you dont have enough power/cone surface x xmax
edit: just noticed that eminence has the peak displacement volume in their datasheet, so i didnt need to calculate it. my values were correct/close enough. their other 15" speaker had a higher xmax and could displace 411mL. eminence legends 10" speakers could displace around 200mL due to higher xmax values, but their sensitivity was around 92/93 (as opposed to 96/97), which is low, and a bad thing when you are looking at more volume. many 15" speakers will have higher xmax values than those of the eminence 15" speakers. the 15 (
http://www.warehouseguitarspeakers.com/proddetail.php?prod=15bass) i have in my cab has an xmax of 7.5mm (compared to 4.7 for the higher eminence speaker) im sure there are others with higher ratings as well, but i dont know much about what is available.