Brace yourself. I started typing and ended up writing a novel.
This biggest thing you need to check with any Randall if you plan to use Synergy modules with it is if it already has the Dual Channel mod or not. If not you can only use one channel on the Synergy & dual Egnater modules. I think Rob/Jaded Faith and Yuneo still sell the midi board to do the mod. One thing to note, Dual Channel mod is only available for the RM100 and RM4. If you've been in an amp before it not too hard to do it yourself. The biggest pain is all the screws to get cover plates and such out the way. Last thing on this, for whatever reason the Diezel Herbert module doesn't work with Randall gear regardless of the dual channel mod or not.
The next must have mod is the Foglifter. This is the one I think you're referencing. This replaces the Presence and Density circuit and helps with clarity. This one is easy to do. Literally unplug the old circuit and plug the new one in. I would personally recommend taking it up to the next step and getting the Foglifter Plus. This adds a negative feedback control. Being able to adjust the NFB is for me the most useful mod in getting everything to sound and feel right. Note on this mod: The RM50 & RM100 you can do the Foglifter or Foglifter plus. The RM20 & RM22 only the Foglifter Plus is available. Obviously no Foglifter on an RM4 since there's no power amp.
For the other Randall amp mods...
-3 way cathode switch: This one is pretty useful for selecting either "Fender" "Marshall" or "Boutique" input bias. On the Randall stuff "Fender" is the default. This mod does affect the tone a little, but has more effect on the feel. It's more like a 3 way tight switch than anything. It is an improvement but not a 100% necessity.
-FX loop Send & Return Controls: This does what it's supposed to, but it doesn't actually replace the loop. You get the adjustments, but the Randall loop sucks ass and is noisy. Because of that this mod really isn't worth anything.
-Input gain: I don't see it on the list of mods offered anymore. That doesn't matter because it's not worth it and I find it's almost useless. All it does is bump the signal level going into V1. It's okay if you use low output pickups, only clean modules, and want to push things to edge of breakup. Otherwise all it does is make things fart out.
-Mercury Magnetics Transformers/choke upgrade: Upgrading the transformers does make a big difference in the sound and feel to amp. I have 2 RM100's one stock transformers and the other with Mercury. As stock there's nothing wrong, but the Mercurys step it up to the next level. They make the amp sound and feel how it should have from the start. That said, the Transformer/choke set is $900 plus costs of installation if you need a tech to do it. Back in the day it was worth the price since there was no Synergy option. At that price now, it's not really worth the cost of admission.
One last thing about the Randall (at least the RM100)... You can use most any big bottle octal tube in it; 6L6 EL34, 5881, etc. will all work. You're able to bias each tube individually as well so having matched sets isn't as crucial.
For the Synergy amps, they surpass the Randall amps in most ways. Synergy is default dual channel, has auto input bias based on the module that's selected, DI output, an infinitely better FX loop, much better clarity, etc. etc. It's also backwards compatible with the MTS modules. Basically, more advanced technology and sounds great off the shelf with no need for mods.
****Major Note***** DO NOT use 1st Gen MTS modules with synergy equipment. They will fry your Synergy amp/preamp. It's a known flaw, but it isn't advertised at all. I unknowingly had this happen to both of my SYN2's. I directly spoke with Rob DeAntonio and Bruce Egnater about this when trying to figure out what happened. Once I gave them the S/N of the module I was using when it happened, Bruce knew exactly what the problem was. The physical size of the module board in 1st gen MTS is slightly different than everything else. This causes the pins to not seat fully in the Synergy dock. That leads to arching across the pins and fries things inside the amp/preamp.
Synergy also announced the SYN20IR which adds IR's, [I think] built in reactive load and some features to the power section that match it to the module being used. I don't know if that will be fully functional with older Synergy modules. The new Z-wreck and Tone King have circuitry built into the module that interact with those power amp matching features. There's internet rumors of maybe a SYN50IR, but nothing confirmed. And if there is one coming, it's probably a good ways off.
So now to the question at hand; which is superior/which one to get? Well the answer is it depends. In pretty much every way but a select few, Synergy is superior to a stock Randall. The couple of things Synergy falls subjectively sort on compared to the Randall is no 100 watt model and only uses 6L6 tubes. If you're looking solely at features between a stock RM100 vs a SYN50, it's Synergy all the way.
It gets a little more hazy once you factor in price and your needs. Let's take a couple examples of a RM100 for about $600-$800 no modules or $1100-$1200 with modules [these seem to be the current going rates regardless of the mods on it] compared to a new SYN50 for $1600 plus another $400 for a module.
Full stock RM100: You'll want to add at minimum the dual channel and foglifter/foglifter plus mod to it. That'll add $300-$650 depending on if you can do the mods yourself or not. And the price gap closes with additional mods to the point of the Randall possibly being more expensive. I'd be hard pressed to pick the Randall then sink more money into it. In this case I'd likely go Synergy.
Partial modded RM100: This is going to be dependent on which mods are on the particular Randall you're looking at. If it doesn't have at least the dual channel mod you're almost on the same page as if it were stock. In that case you may as well go Synergy. If it has at least the dual channel mod and base foglifter you're covered for the essentials. The Randall becomes more appealing vs Synergy when you don't have to put more money towards it. At worse you may want to add the cathode switch. [If you can DIY it's a couple dollars for a DPDT switch, 2 resistors, 2 caps, and some wire. Build the switch then connect 2 leads from the switch to V1.] If you don't need extra features of the Synergy I don't think it's worth double the price of a Randall. As long as it had the essential mods I'd pick the Randall. I'd also give more consideration to the Randall if the only mod it had was the Mercury transformers.
Fully modded RM100: I'm calling fully modded as having at minimum dual channel, foglifter plus, cathode switch & Mercury transformers. In this case I'm going to pick the Randall every time. In fact this is exactly what I did choose. These mods; especially the transformers, put the Randall on par with Synergy. At this point the only thing Randall lacks is a good loop and maybe auto cathode bias. With most other things being equal, three module slots, 100 watts and a cheaper price tag gave the Randall the advantage for me.
The one thing I would say that would make Synergy beat Randall in any situation is if you absolutely need the FX loop. There's no amount of mods, including the Mercury transformers that could overcome that deficiency. The RM100 loop is bad enough that it's almost unusable. And I don't know if aftermarket loops are compatible.
I'm going to stop here just comparing the amps. There's a different set of factors if you're deciding between SYN2 and RM4 preamps. Do let me know if you want me to go over that and I will.
I know that was a bit long winded, but hopefully you got something useful out of that helps with your decision.
