Adcom 555,B&k St 140, Mccormack .5 dna

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.

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.