A single set of midbass drivers around 60hz and up on 100w rms, or two on 50w rms each with my crossover a little lower around 45hz. Which will give more output?
Two woofers will sound better in general, but I think it's pointless to move the crossover so low. You should almost certainly use a subwoofer to fill frequencies below 80Hz, and most certainly below 60Hz.
First, I think the rule of thumb is that doubling the cone area on the same power gives you +3dB, so the later will be louder. However, doubling the speaker's cone area will not change the fact that they are rolling off in the midbass region. Take a look at this picture (
http://www.avtozvuk.com/az/2009/06/Comp%20gr/Comp_gr_polk.jpg). This is measured frequency response for Polk MM woofer. It's rolling off sharply below 100Hz. This is typical of most 6.5 inch speakers. Granted, at the limit, the two speakers will be less likely to bottom out, unless you take their crossover frequency to like 45Hz.
IMHO, one rule of thumb for midbass is the following. Set your high pass filter no lower than the resonance frequency of the woofer. The resonance frequency of Polk MM speakers is at about 80Hz, so I suggest to set your high pass at that frequency with whichever slope works best for you.
I'm asking because if I give rated power to my current polks, I have to up the crossover a lot. If I have them running on a lower rated amp, I can up the xover but output is weak.
This is why speaker's RMS power ratings are often useless. The rating is usually done for thermal power handling, but most 6.5 inch speakers will run out of usable excursion if their crossover is set low enough. A typical "100watt RMS" speaker will bottom out on 50watts of power with a high pass crossover low enough. 80Hz is enough for a lot of speakers if you send something like resembling rated RMS wattage to them.
So my point is, two woofers will sound better, but there is still no need to take their high pass frequency so low. If you have a subwoofer, the midbass woofer's overall contribution at 45-50Hz is pretty much in-audible. You have a problem if your subwoofer does not play loud in that region IMHO.
If you want the front speakers to play more bass and handle more power, take a look at Massive Audio CK6. Everyone says they can handle bass pretty well.
