Upgrade only one item at a time or you won't know the actual impact of the individual items. Speakers generally have the biggest impact and are the easiest to hear a change in the sound quality and should be upgraded first. There are a ton of great speakers out there to choose from between $5 and $10K.
After that and only after that should you consider upgrading your integrated amp if so desired. Allow the speakers to break in per the required time and then see how your current amp and speakers actually perform together as a system.
Your PS Audio Strata Integrated is a very decent amp at that price level and the rule of thump is you have to spend 2x the list price amount to get a noticeable difference but different amps could also have different sound characteristics so that would also come into play in spite of price.