Running into a traditional amp, you'll generally have better results if you don't use any cabs, but units like the DT25 (and any other modeling combo-Fender Mustang, BOSS Katana, etc) are designed to NOT allow much coloration from the power amp and speaker (that is all handled in the models). So with the Line 6 amp it's no surprise that running just a preamp block sounds a tad off. Set it up the way it sounds best to you-that is the ONLY rule in modeling.