In this work, contrast-variation small-angle and ultra-small-angle neutron scattering are used together with wide-angle X-ray scattering (WAXS) to characterize the bulk molecular conformation and self-assembly of polythiophene-based conjugated polymers (CPs) in bulk blends with deuterated polystyrene (PS-d(8)) as the matrix component. A significant and sharp transition from small to large globular domains is observed in the phase-separated morphology of all blends as a function of CP concentration. Evidence of self-assembly into nanofiber networks is also observed in regio-regular poly(3-hexylthiophene) (RRe-P3HT) blends and found to be promoted by the use of solvents of moderate quality (i.e., toluene) during the film formation process and by higher CP loadings when using solvents of good quality (i.e., chloroform). Finally, WAXS and conductivity measurements demonstrate a strong correlation between the degree of crystallinity of the CP in the pi-stacking direction (nanofiber formation) and the electronic conductivity across the bulk of the film. In addition to RRe-P3HT, PS-d(8) blends with semi-crystalline poly(3-dodecylthiophene) (P3DDT) or poly(3,3'"-didodecyl[2,2':5',2 '':5 '',2'"-quaterthiophene]-5,5'"-diyl) (PQT-12) and blends with amorphous regiorandom poly(3-hexylthiophene) (RRa-P3HT) were investigated over concentrations ranging from 0.1 to 50 wt % of CP. This work highlights the importance of understanding the factors that influence the phase morphology in blends of CPs and commodity polymers, as this directly alters charge transport pathways and performance of the organic electronic devices that rely on these materials.