[quote name='Wiggybass' timestamp='1349207930' post='1823389']
No reason whatsoever why you shouldn't mix any speaker sizes you want so long as you don't go below the minimum impedance (stated in ohms) that your amp will safely drive. Going below that point will make your amp work very hard and likely shorten its life. Most bass amps I've used seem to be happiest working at 4 ohms, and will typically deliver about 25% more power than when working at 8 ohms.
What size you prefer depends a lot on how you use them and what music you play. Fifteens react slower (marginally) but will reproduce lower frequencies (typically regarded as those below about 160 Hz) more readily, so can be good for the deep bass required in dub, reggae, house etc. Tens react faster (marginally) and will not 'go as low' as larger speakers, simply because there is less cardboard moving in and out, but personally I prefer them for reasons of both sound quality and portability. I used an Eden 2 x 10 plus 1 x 15 setup for a while and it was OK, but since I bought my first 4 x 10 I've not really used anything else.
By far the loudest rig I've owned was an original Trace Elliot 300w head with two T-E 4 x 10s, which, despite the modest power of the amp, set off car alarms 50 yards away at half volume outdoors...
[/quote]
Sorry, but quite a bit of that sounds like you've been filled with marketing codswallop.
The main reason you shouldn't mix driver sizes is that you'll get unpredictable results from venue to venue.
As for 15s producing lower frequencies, it's just not true. Marketing gobcrappers would have you believe that, but if you go for decent drivers from the same manufacturer, the specs on how low a frequency it can produce have no baring on driver diameter & more so on how the cab has been designed.
keeping to the same size drivers means you'll get a consistent sound from venue to venue.
I'm not saying don't go for a 4x10 & a 1x15, but you won't really be hearing fully what the 15 is doing as the 4x10 would probably be drowning most of it out.