The newest combo w/d will fit best in the 3670 and it is my understanding they work great but don't know if they will wash a large load like the stackables (king sheets). Our original BH had a smaller combo that wouldn't hardly wash more than four pieces at a time and we had to go to the laundromat for sheets, jeans and anything large.
We LOVE the stackable in the Augusta and was one of the primary reasons we chose the Landmark. I wouldn't have anything else but stackable now.
K&C RV in Longmont, CO says 3670 is not designed to accept the stackable; however, Camping World, Katy, TX, says their techs have done it - takes some modification of the closet interior. I would investigate with service department first to see if they have capable, experienced techs. If it has been done by your dealer, can you talk to the customer to see how they like the mod? Just something to consider.
Good luck.
CJ