Most Sony HU are junk. Although, I don't think that it neccisarily means that you subs won't be as loud. What they meant was probably that Sony HU dont have as high of preout voltage for the amplifiers as nicer HUs like Alpine or eclipse do. In any case, if you don't already have a Sony HU don't buy one.
I would have to say most headunits have only 2v pre-amp outputs. Going from 2v to 4v isnt that much of a difference. Your amplifier for your subwoofers would just have to work harder to compensate for having lower pre-amp outputs. So to answer your question, I would say no because it all depends on the amplifier you are using for your subwoofers.
You will have to turn the gain up on your amplifiers a bit.
As far as Alpine goes, the new Alpines are hardly better than Sony.
sony the higher models arnt bad at all from my experiance. and it doesnt matter what the pre out voltage is just if you set your gains right. and alot of pioneers and alpines pre out is 2v also.
