The McCormack .5 with an Audible Illusions preamp. This will easily compete with the newer offerings from most companies. The other two amps are not in the same league.
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I have two GFA-555II's, and they are very powerful, and will handle a 4 ohm load with no difficulty. They are pretty strong on bass reproduction, too, but my guess is that sonically, the very excellent McCormack DNA-.5 will be noticeably superior. Interestingly, I will (hopefully fairly soon) be comparing my 555II's to at least amps from Krell, McCormack and Theta (and possibly others) as I contemplate an upgrade for my 4 ohm Thiels. The Adcoms drive them just fine, but I am looking for a better sonic synergy match.
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.
I have a 555 from the late 80s. It handles the difficult loads very well. There was no problem with Thiel 3.6 nor Thiel 2.3. However, the 555's sonics are mediocre at best compared to the so many great amps of today. The Adcom simply collapses the music into a 2-dimensional presentation and has only-ok resolution. I really had no idea how poor this was until I cycled it through my audio system when I was trying the ARC VT130, ARC Classic 150s and then the very impressive Counterpoint NPS 400. Talk of the 535II being the great Adcom amp in the lineup was quickly dismissed as this too had the same qualities as the 555.
At its time, the 555 was a great value for midfi systems. Today it is really only suitable for a non-serious 2nd or 3rd system or as a starting point for a Home Theatre application. I currently use it in the latter with good results until I get serious with some Pass or Plinius amps here. I am sure the McCormack DNA series would be a major improvment over the 555.
There is no doubt about the Adcom driving a 4 ohm load. It will do so with ease. I don't know too many audiophiles that have not owned an Adcom at some point. I used a 555 way back when.
The difference is in the sound. This is where the McCormack will jump over the other 2 amps. It has much better soundstaging and transparency. It also has one of the quickest and most pure bottom ends around. At its current used price of around $600, it is a absolute steal. Then, later on, upgrade it with McCormacks performance enhancers.
Depending on your speakers that you are using, the Belles 150a Hotrod amp is one of the purest sonically. It is not the last word in transparency or dynamics but it sure makes music. It can be had used at around $700.