It depends on which Pass amps you're talking about?
I run X250.5 with Audio Research LS-25 tubed preamp with B&W N803 speakers. Very nice.
So I guess it will also depend on which preamp you will pair the amp with.
I heard my amp with Totem Mani-2 Sig and it was excellent with Audio Research tubed pre.
I think Totems benefit from more power, so you may want to cosider Pass X250.5 amp if you also want more power than Plinius.
Pass X.5 amps are smooth sounding. I've read that the X.5 series is supposed to be smoother than the previous X series, but I have not heard the previous amps.
I've heard Plinius but not in my system. Sounds great paired with the right speakers. What is your goal as far as sound? What are you looking for from the amp?
In any case, either amp paired with a good preamp will be a nice step up from your integrated, IMO.
Pass will probably be a bit more neutral and dynamic but it will not be cold sounding, if that's what you're worried about. Also, with Pass you have transferrable warranty if you buy used, as long as the amp is not older than 3 years. I don't think with Plinius you have this.