Can I use a 4 channel Amp to run 2 6x9s and a 10" sub? Amp is 80 x 4 / 250 x 2...would bridge the sub and then run 1 6x9 on each channel....Would the sub get 250 and each 6x9 125?
This is assuming, however, that the sub in question is either a single 4 Ohm voice coil sub or a dual 2 Ohm voice coil sub that can have the coils wired in series to a 4 Ohm nominal load.
This is because the majority of amps are stable no lower than 4 Ohms in mono bridged configuration and that would be what you'd have combining 2 of those 4 channels into a 3rd, more powerful output.
Outside of that one restriction jmanpc is absolutely correct.
80w each to the ByNines being driven by the 2 channels in 4 Ohm stereo mode and 250w to the 10" being powered by the 3rd bridged output.
What do you think of the Polk Audio db104 -- Its an SVC that handles 270 RMS...I can get it for 50$ w/ free shipping...plus I plan to use the polk db series as the 6x9