The sub absolutely makes a difference. I'm sure you are hearing frequencies that the Carvin can't reproduce. I'm also sure that the Carvin will go louder without distortion than the Logitechs. The Logitechs are made for home entertainment or computer systems but are quite good is small rooms. I'm sure there will be situations in which you will find they don't quite do the job. Also, you will need to be careful when moving and setting up the Logitechs as the little wires and connectors are really designed for this type of use. I suspect they will be pretty susceptible to damage.
To get the same quality of sound in larger systems is quite expensive, as most of the top-end, self-powered, bi-amped units run in the $800/ per side range.
I was just on the phone with Hank The Lurker telling him how great his Barbetta 31Cs are. I've been trying them out, and I'm seriously thinking about switching from my 32Cs. I can't tell any difference in the sound, and his are smaller and a little lighter.
I believe the 32Cs would be better in really large situations, but 95% of my jobs are small. I plan to hook both sets up and experiment a little before deciding.
DonM