You have received lots of informative comments and interesting viewpoints to your question. Having recently gone through an amplifier search/trial I can attest to both the impact an amplifier can have on the sound of one’s system as well as the importance of personal listening preferences in choosing "just the right amp" for your system. Even between several very good and highly regarded amplifiers, minor sonic characteristics can easily affect your level of satisfaction in how your system sounds.
Regarding your three listed amplifiers, they all have their supporters and are all highly regarded amplifiers so the questions to answer are, first, are they all capable of delivering the level of dynamics you desire with the speakers you plan to use and, second, which of them sound best to you on the types of music you enjoy most.
The D’agostino and PS Audio amps should drive just about anything. As some have pointed out, the ability to double output when the impedance is halved is a desirable quality because it allows the amplifier to drive difficult loads with a minimum of frequency and output aberrations. Of the three, based on the posted measurements, the D’Agostino amp should drive about anything you would throw at it and should exhibit a tight fisted control of the bass. I suspect the PS Audio amps will also work well with most speakers but I recommend reading JA's measurements section in the Stereophile review of the PS Audio amp because he concludes those amps may not work so well with speakers that have impedance dipping below 4 ohms. I suspect the Modwright amp will not equal the other two in controlling a wide range of speakers because it has less power to begin with, and does not double power into even 4 ohms. For those reasons, unless I could confirm that it sounded excellent with a specific pair of speakers I was planning to keep, I would not consider the Modwright.
Some other differences to consider between the PS Audio amps and the D’Agostino, are,
1. The PS Audio BHK Sig. 300 amps are monoblocks, which I prefer both for separation of the power supplies and also because of the ability to use very short speaker cables,
2. The PS Audio amps have a tubed input stage, which means you may deal with minor tube noise and will have to replace tubes. This is not a big deal to me since you are only looking at two tubes per amp, but some may not like having that extra layer of maintenance and operational responsibility, particularly people who like to leave equipment powered up all the time,
3. The PS Audio amps use mosfet output transistors while the D’Agostino uses Sanken bi-polar output transistors (which I liked the sound of in a pair of Cary MB500 monos I once owned). In general, I like the sonic signature and control of bi-polar devices. In my personal amplifier evaluation, I was able to own and compare at the same time a pair of Lamm M1.2 Reference amps (that use an input tube and mosfets) against Clayton M300 amps that use Motorola bi-polar output transistors. I ended up selling the Lamms because I simply did not like the sound as well. Similar to a BAT amp I once owned that used mosfets, I found they made the Lamms sound comparably "soft" and particularly in the low frequencies (in fairness, the Lamms only output about a third of the power of the Claytons, which likely also had an effect). This possibly "softer" mosfet sound may be reflected in the Stereophile review of the BHK 300s where Michael Fremer says, "The BHK 300s’ bottom end is nimble, well textured, and rhythmically solid, though there’s still more weight, power, and physicality to be had in the very lowest octaves."
You should search out the reviews of these amps, however, I would caution you to take them with a grain of salt since the reviewer’s systems and listening preferences are not necessarily your own. As an example, although both writers wrote positive reviews of the D’Agostino Classic, Jacob Heilbrunn said, " If this amp has a weak spot, it’s the treble where it can harden slightly on very complex and dynamic passages," while Ken Kessler’s review did not find any problems with the treble and said, "The Classic eschews sibilance unless the recording is poor." Both reviews are very positive and, in particular, Kessler’s review speaks very highly of the Classic.
My strong suggestion is to listen to the amplifiers you are interested in before purchasing, and preferably listen to them in your own system. I ended up auditioning four different amplifiers and had to purchase all of them to do so but, in the end, I was able to distinguish clear differences and to choose the amplifiers I liked best. Good luck and let us know how you make out.
