Well, it really depends on your definition of "substantial difference". What one considers substantial may be minute difference in someone else's opionion. Let's start with the obvious, what do you find lacking with your current Plinius? What are you looking to improve on? Have you read the reviews of the SixPacs (and its upscale sibling, Cary V12) on-line? There's one at soundstage.com which compare the SixPacs with SimAudio solid-state amplifier if you have not seen it yet.
I have a pair of SixPacs in my system and am quite happy with them. I think they are more musical than my Classe' CA300, ARC VT100, but that's my preference. You'll need to listen for yourself, and I understand that's gonna be hard to get a sample pair in your system.
Good Luck.
Spike