That depends on your taste. I have personally wanted badly to switch from tubes to solid state, and I've tried numerous times, but always find something missing when I do. Others seem make the switch comfortably, or not, depending on taste.
This is not an easy decision, totally subjective, but I do think the Proac is a noticable step up from the particular Spendors that you mention. And I think the CAV-50 power and sound character rather limits your choice of speakers. For example, you could probably power many of the Silverline speakers comfortably with the CAV-50, but I'm not sure the sound qualities would complement each other (they might).
I have always found ARC gear to be a little bit too lean for my tastes, but I've not heard the latest generation.
The Proac 2.5 definitely benefits from power. 100 wpc might be plenty for a small room but marginal for a larger room. The high water mark for sound quality in my home was when I had the Conrad Johnson Premier 12's (140 wpc, tube watts) and was switching back and forth between the Proac 2.5's and the Spendor FL-9's.
Sigh. In solid state, a good value is the Plinius 8200 version II. That would be worth trying out with the Proac 2.5, as would the ARC I suppose. The Plinius needs a heavier nonstock power cord and also 24 hours to run in.
Art
