Well my answer is hypothetical since I never hunt in waders and don't even own a set. BUT...I do own some base layers from Simms and can tell you there is (for me) very little out there that surpasses the Simms Guide Mid Pant and Top as super-warm and comfortable insulation layers. The pant is $79 and that is cheaper than an average pair of merino base layer pants. My Mid Top by Simms has quickly become one of my all-time favorite grab-and-go warm tops.
You didn't mention what conditions/temps you're hunting in, how far you expect to walk in them or if planning to stand in cold water for long periods of time. These are factors that do into my decision making and recommendation. I don't wear pants under my waders. Kind of feel like, what's the point. Very little warmth value, often bunch up, etc.
In warmer conditions or where I might need to walk a lot in the waders, I'll wear light weight merino long underwear. Synthetics are fine too, but wear them for more than a day or two and merino is better (smell, crotch rot, etc). If hunting in late season, cold weather and standing in water....then I go a heavy fleece pant meant for waders. If really cold I'll go long underwear under the fleece. I do a lot of steelhead fishing all winter, often in nasty conditions, so I take my cold weather waders pretty seriously. The best fleece pant I have found are Redingtons. Very thick fleece compared to what's out there today and they have a thinner fabric in the ankle with stirrups. Very warm, comfortable and won't bunch up to constrict circulation. I can spent 8-10 hrs fishing in very cold weather and not get cold. Use the same setup for late season duck hunting. But I wouldn't want to walk too far in them. You can overheat pretty fast.
Thanks for all the replies. I have some Kryptek merino base layer bottoms and some Russel APXG2 fleece pants that sound like they should work.
As far as conditions I really don't know. I'm going hunting thru Papa Bear for 10 days, 9-19 thru 9-29 in 2018. I don't think we will be doing a ton of walking from talking to others that have gone on the trip. Weather is always an unknown, as is the actual place he will set us up. Without knowing the area I'm just trying to be as prepared for various conditions as possible. It sounds like there is a good chance we will spend any time away from camp in the waders. I hate to pack several pairs of heavier hunting pants if I just need base layers to go under the waders.
I've been spending a lot of time alone on the horse checking cattle or the tractor haying and had a lot of time to think about the hunt. I've been listening to podcasts on my phone in the tractor getting more ideas too. Trying to get things dialed in a bit now while I have time so that I'm not stressing at the last minute.
I been steelhead fishing a bunch this year. Depends on the temps...but I am running the pocketless guide version of the G3, really like them better then my originals. Anyway for bottoms I run wool power Merino bases, then a Sitka heavy weight grid fleece bottom.
On top I run an R1 Patagonia hoody, then a sims polar tec sweater over it. For outerwear I either run a puffy of its dry or if it wet then I run a puffy vest and G3 Jacket.
Seriously. I can’t get even get a season of fishing out of them. For hunting happiest I could get was Hodgman or Patagonia over merino light or midweights and carharts over the waders. Sims g3,g4 g37, g692 are too thick and heavy for this treatment but even so DIDNOT DO NOT hold up to breaking brush or hiking into holes way back away from it all etc. it’s cool- just pinholes- just like a drizzle.
The others don’t hold up to abuse either but are light enough to throw the carharts over. Actually valdez vans aka xtratuffs and not going in mud or water over the tops of them was my favorite for moose hunting. But either way merino bottoms underneath. The smell that starts coming out of waders after 10 or so hours when they contain a dirty human and some synthetic Long Johns is potent and the waders act as a billows to keep your nostrils full...