Reddit mentions of Champion Men's Tech Performance Boxer Brief, Black, Large

• These are inexpensive, pack of 3, if you zoom in you can see the seams are out of the way. They're fairly soft. However, they don't keep you quite as "dry/cool" due to being 50% cotton, and they lose their shape/elasticity slightly over time.
  
  
• This is 92% polyester 8% spandex, it holds it shape perfectly and will be very cool/dry. However, it does have a middle-seam, but personally I don't find it uncomfortable. That might vary person-to-person. Slightly more expensive, only 1-pair.
  
  
• This is 91% polyamide/nylon, 9% spandex. It will keep you dry/cool, but unlike the 92% polyester above, it will feel much softer than any other blend. Holds its shape perfectly. These types cost more because they feel softer and they look sexier. The nylon/spandex types are more skin-tighter. But if that sort of stuff doesn't matter to you then don't bother I guess.
  
  
• This is much like the above, modal/lycra is nylon/spandex. The one pair that I have doesn't hold its shape quite so much but it's a looser fit. It feels extremely soft, but there is a seam. Causes no problems for me, though. But I don't like the waistband logo, so I do in fact prefer the above, because you can cut off the Jockey tag and there's no logo anymore.
  
  I'd start by solving your seam-chafing problem and go from there. It's hard to say for other underwear because most trunks/boxer briefs do in fact have the mid-seam, but the difference is that they do a much better job of supporting your junk and keeping it in place, so less movement= less chafe. And regardless of seams or not, they will probably feel more comfortable so long as you pick good stuff.
  
  Obviously you could spend more time shopping about, but those I personally own and like. But no two dicks are the same. Everyone should be picky about their underwear. But, I'd go with #1 since you're used to boxer shorts.

Extra long boxer briefs for the win!