There is your problem. It seems like everyone has a 'friend' that feeds them bullshit because the friend doesn't know what they're doing, so kudos to you for posting to get other opinions. And from the sounds of it, I wouldn't listen to a whole lot that the said friend has to say.

The dual voice coil subs would actually work better off of the amp you linked. Issues do not arise by running dual voice coil subs from one amp, issues arise when the voice coils do not match the final impedance the amp needs. With those dual voice coil subs, you would have a final load at the amp of either 2 ohms stereo or 4 ohms bridged, either way getting you more power out of your amp than the single voice coil 4 ohm subs.

To put it simply, with the first subs you linked you would get 170 watts rms x2 from the amp when the subs say they handle 300. With those dual voice coil RF subs, you would get 250 watts rms x2 from the amp when the subs say they can handle 250.