Short List of Amps I prefer over the Pass Labs XA25 or INT 25

Pass Labs admittedly and intentionally adds some even order harmonic distortion to all their amp designs. This is commonly known to be perceived by humans as a euphonic overall sound quality similar to tube amps, which many devoted Pass fans and others desire and enjoy.

Be that as it probable is, it’s probably more true of Pass’ First Watt than of his Pass Labs amps. In any case, I believe it’s well established that the FW amps are primarily marketed to those with high efficiency speakers; 93db and above, and not falling much below 8 ohms. And while I can’t swear to it, I think that typical distortion levels from at least some of the FW models driving speakers like those are respectably low. However, I've yet to see much in the way of a full set of independent distortion and other measurements done on most FW amps-certainly not on the F4, which I own and plan to use with my new speakers. The one exception I know of was the Stereophile review of the FW J2, where JA did his usual array of tests. IIRC, the J2 did quite well with 8 ohm loads.

I don't quite have enough experience to be sure that this is the reason, but both the Coincident 300b Frankensteins and the First Watt F4s are dominated by 3rd order harmonics, while the XA25 is dominated by the 2nd (and all three have low levels of higher order harmonics).  My guess is that my ears are sensitive to the balance between the 2nd and 3rd, and prefer amps that are 3rd harmonic dominated. 

About the F4's distortion, Nelson Pass said ".....the distortion is 2nd and 3rd harmonic in character, rising or declining in linear proportion to the output
power." I wish there were actual distortion vs. power measurements done on a stock F4- at least at 8 ohm loads-that showed where most of the harmonic distortion content was during the first 10 watts. Like your Coincident Pure Ref MK2's, mine are 94db, 8 ohm and never drop below 7 ohms, though they only play down to 70Hz.