Is It Worh Modding An Amp

In my opinion this would depend on what is behind it.  You'll notice dramatic changes in sound be modding your source (if necessary) as well as any pre-amplification you may be using.  But modding the power amplifier should be the last on the list unless it has some well-known draw-back that should be addressed.

If you have to ask then probably not. 

I had Ric Schultz mod my GaN1 and it sounds terrific,  but if I had to sell it I wouldn't recover any of the $500 the mod cost.  It's a risk.

I'll add some more.

If I need a repair of any precious equipment I know techs who only will replace non-working components and touch or re-touch nothing else if everything else is in tact. 

I've repaired already several vintage and modern components at reasonable prices paid mostly for labor and diagnostics and rest little for parts and their quality work brings many units back to original specs like fresh new strings on guitar or violin. 

They don't offer any upgrades for the sakes of keeping solid reputation. Far more than often those upgrades don't satisfy customer and require lots of uncreative labor. Real pros would never advise you to fall on that upgrade bait.

@treepmeyer Ugh. I sent something to Ric once and he sent me back garbage. Horrible soldering, bad attitude and bad treatment of my materials. Avoid this turkey and learn to DIY.