I dont think modifying a modern pcb constructed 100 watt amp like yours to get "sag" is a good idea at all. Here is why:
1. It was never designed for that in the first place. 100watt amps like that are built for punch and headroom. Exactly what are you hoping will happen? That it will turn into a tweed deluxe but with the roar of a jcm800? You are really trying to turn the amp into something it will never be. Let the amp sound like it was designed to sound and enjoy it, or sell it.
2. It is built on a pcb. These kinds of amps are very difficult to work on. Even just getting to the parts you need to work on is time consuming, fiddly and the construction using cheap board mounted components is notoriously fragile. What this means is that it is not an easy job for a tech to work on. Time=money. You can easily end up spending more than the amp is worth in labour time for what may or may not even get you close to your ideal sound.
3. There is no way of knowing if the mods will do what you are hoping for anyway. It is pretty invasive and there is every chance you will just neuter the amp and make it worthless.
If you want an amp that sags, or has less headroom, more spongy feel or whatever..get one. Don't try to force a square peg into a round hole.
If you really do like the jcm800 style roar of the winsdor, but just want less sheer volume, a more vintage vibe and perhaps more compression, then you would be much better off spending your money on some greenbacks. I dont know what speakers you are currently running, but grenbacks will definitely cut your volume down so you can push your tubes a bit harder. They will compress and offer a more forgiving, more old school style of tone. Dropping 3db is equivalent to dropping from 100w amp to a 50w amp. They will have less bass defined than most modern style speakers..and voltage sag is mostly noticeable when amplifying low frequencies. Low efficiency speakers with a loose low end like greenbacks will give a somewhat similar effect when being pushed hard and let's face it...you are only imagining a particular tone and feel in your head right now, at least a speaker swap is easily reversible. Greenbacks or greenback style speakers are plentiful and not particularly expensive. I'll bet that for the amount of money you are thinking of investing in tech fees and parts, you can probably buy 4 speakers. Worst case scenario is that you dont like the change, but still have four speakers that you own and can sell if need be rather than having a chinese built amp that has been totally butchered inside and has no resale value. I would bet though, that running a quad of greenbacks (or cheaper clones like wgs green berets) will get you in the ballpark of keeping the good parts of your amp tone, but changing it to a smoother looser more vintage feel that will sound killer.
