Any suggestions for reels (prefer new, USA made but open) the line is a DT8? I like 100 yd of 20 lb backing.

Valentine 95.

I like the 1498 better than 1495 1/2 for the DT8F, but I'm not sure I get even close to that much backing on either of them. It's been a while since I saw the backing, but if memory serves it was nearer 50 yds. with fat Peach. Switching to WF leaves a lot of space or backing and I think if I was expecting fish to get into the backing that I'd be using WF or shooting heads. Probably use PE braid rather than Dacron too.
With a DT8S there is lots of room for backing too, but I doubt DT sinkers are truly useful, mine never were.

Those Medalists are rock solid, for sure. If you're wanting new and USA-made, have a look into Galvan reels. I've come to like them quite a bit over the past few years. The Torque (T-8 or 9) or Rush Light (R-8 or 10) might suit you. Their website specifies backing capacity for wf lines but you could reach out to them to ask about DT lines. They're a small, family-run outfit that makes a solid product and provides outstanding customer service. Check 'em out: https://galvanflyreels.com/reels/

Have fun and good luck!

Another vote for Valentine , maybe an earlier Ross Evolution or earlier Abel made or Madison Orvis stuff , everyone’s earlier suggestions will suit nicely …

Highly recommend Galvan, especially the Rush Light. If your budget is lower go with a Lamson Remix.

Galvan all the way. They’re go-to reels for guides because they can take every-day-in-the-boat punishment for years. I have a Torque set up as my big-line steelhead reel, but the Rush Light is an awesome option too…

I used to have a Battenkill 8/9 Disc Drag reel that held a whole lot of backing with a wf8f on it.And I mean a lot of backing.

You mention new, and 100lbs of 20lb backing, but don't mention what you are fishing for. I would put at least 30lb dacron on, or maybe 50lb specta/braid and go ahead and load it up with 200yards, just in case you want to put it to use in open water where a fish can really run. If you are fishing for Bass, then it really doesn't matter, they don't run. With a DT8 you've entered the realm of big honkin fly reels, so I would think about the real usage you plan on giving the reel, since salmon or saltwater fish are going to require a lot more than Pike/Musky/Bass.

For reels that big that are saltwater capable, I usually look on the used market. There are a lot of people who buy reels for saltwater trips and use them once or not at all.

I would go for a Ross Evolution R Salt 11/12 or similar for the large arbor and spool space. And a DT8 is going to fill up most modern reels rated for a 10wt (since they are rated for a WF 10).
If you are looking for something that isn't a large arbor, an old Ross Gunnison G5 or Salt. I have old Able Big Game 3 and Big Game 3.5N reels that fit the bill. The Abel Big Game 2 will put a DT8 on but I'm not sure how much backing it will hold. However Spectra saves the day here, since 40lb spectra is thinner than 20lb Dacron.

The Valentine is good for an old reel, and there are some nice, beautiful traditional looking reels made for the Spey market.
If you are just fishing for bass, a Medalist 1495.5 or 1498 will work. As will larger click and Pawl reels, as long as you can palm them. Young made some really nice reels, and I'm not sure which sizes are good, but I would guess a 1535 would be nice.

Maybe Bulldog can provide a good size JW Young reel for this use. If I were going Click and Pawl for Salmon, I would probably go with Young.

Sorry for the rambling, but I am shopping vicariously through you.