Is It Worh Modding An Amp

I would just roll tubes. Easy to do, easily reversible, many combinations available between all the preamp and power tube combos, won’t hurt resale, plus the unused tubes can be kept for future amps or just resold.

The Dialogue 1 uses pretty inexpensive coupling caps and shunt filtering caps on the input tubes. There are much better caps out there but they are not cheap. The problem is most of these caps are considerably large in size than the stock caps so are not going to fit on the bias PCB. They will have to hang somewhere. Where are you going to place them? They will help the sound a bit but what is the point if you are not using the amp. Would be better off selling it and moving on to something else. Have owned most of the PL line from the very early models to the later models. 

I vote no.  There's not much to mod, maybe 6 film caps?  That's about it.  I wouldn't mess with the electrolytics.  The limited factors in any tube amp are going to be the circuit and the output transformers, maybe the power supply if it's really inadequate, which I don't think is the case here.  It is what it is.  If it's gathering dust, sell it and use the money for something you can use. ;-)

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.