I have a UGQ Bandit XL top quilt and it’s awesome. I’m sure their underquilts are equally warm but I was trying to keep costs down when I put together my setup so I went with the Arrowhead Equipment Jarbidge underquilt. It’s a 3/4 (more like 7/8) length synthetic underquilt that’s basic but...
I just picked up a UGQ Bandit XL (6’1” side sleeper so I wanted some extra room) in a 10* last year and it’s been toasty warm. Looks like they run about $225-$300 currently depending on options
Another vote for the cascade mountain poles. Mine have been great! Plus I’ve got a $25 bear butt hammock that I use as my sleep system that’s been super comfy.
I just picked up a ‘tall’ mainframe along with the zip in dry bag and batwings. I haven’t hauled around much weight with it yet but The tall gets the load lifters up where they should be so I’m hoping it’ll work pretty well.
Between the dry bag and the batwings I have a similarly configurable...
I’ve run the source system for the last year and a half and personally really like it. With the quick disconnect setup you could always buy a second tube and daisy chain them together...but that might be more tube than you really need.