One amp for sub + 2 speakers
 

I've got 2 polk speakers plus a focal 27KX, which takes up to 300W RMS at 4 ohms. I'm thinking of getting a JL HD600/4 amp, bridging two of the inputs and running the speakers and sub off the same amp. Anything wrong with this?

DO not run speakers off one channel, unless i read that wrong. Each set of speakers meaning left channel and right channel needs their own output from a amp. Maybe look at the alpine pdx5, the jl will work as well just bridge the two channels for the sub and not the speakers.

I'm doing the exact same setup except I have JL separates up front and a single 8w3v3 sub. I'll be driving the entire system with a single JL HD 400. The front two channels will run the separates up front and the two rear channels will be bridged to run the sub.

I think you read that wrong, Yes you can use a 4ch amp as a 3ch. As mentioned ch 1+2 go to the fronts, then ch 3+4 get bridged for the sub. just make sure you turn the X-overs on the amp to the right settings.