Is It Worh Modding An Amp

Major mods to amplifiers are as valuable as you the buyer think they are. If you really trust the person doing the mods and those mods address known reliability or sound quality issues, then I would say go for it. With luck you get a somewhat better piece of gear and if this is done as a regular maintenance/update then its much easier to justify (for me anyway) then doing it 'just because I can.' That said, you can look at this like the used car thing...modifications to a car almost always reduce its resale value unless it addresses a known issue with that make or model. But body kits, things that step far outside the original design, almost always is never a good idea.

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.