Parasound a21 upgrades

Bought a lightly used 2 year old Parasound A21 some days ago. It has exceeded my expectations for sure. Right up my alley warm mids and non aggressive highs but also dynamic and tight with lots of headroom. 
Are there any mods that are reasonable to do to these amps? Fuses etc. Can the bias be safely increased to add more class a watts? 
Also have there been any changes over the years? So is a 2 year old A21 the same as a 15 year old one? 

Thanks