I'd rule out the B&K ST140 for 4 Ohm load. The McCormack's higher in-put impedance would probably make it the most compatible of the candidates for the usually high out-put impedances of most tube pre-amps, just be mindfull of the tube pre's volume control/voltage out-put and the sensitivity of the McCormack. While the Adcom and the McCormack sound different they both fullfill your sonic criteria.
Adcom 555,B&k St 140, Mccormack .5 dna
Out of these older "vintage" amps...which one would mate best with a 4 ohm load? ALso...which one would sound competitive with modern amps in terms of transparency,detail, and bass reproduction? ANd if you were to add a tube pre...which one would you choose?