Just a few more comments about the merits and limitations of Class-A amps driving the HE-6.
My 35 years of dabbling in home stero have left me with many speaker amps. Experimenting with them on the HE-6 led me to a few general understanding of their sound, limited of course To only the amps that I own.
The M2 and J2 are great matches for the HE-6 mostly because of their ultra smooth midrange, micro-details and expansive soundstage, the last two traits in most abundance with the J2. I was told by reliable sources that the F1J performs even better with the HE-6 but it cannot drive speakers with passive crossover which is enough reason for me not to buy it.
Despite all their virtues, these Class-A amps seem to share common sonic traits that are not necessarily synergistic with the HE-6: their bass is tight but lacking slightly in impact (too much control or lack of upper-bass overhang?) and the midrange, while ultra smooth and lovely, tend to sound slightly recessed. I should be careful here: the midrange of these amps are so smooth and free of the usual gunk you hear with most amps that, by comparison, they seem not to be as forward sounding. These amps almost seem to be too clean for the HE-6...
When I switched to Nelson Pass's earlier class-A designs such as his early Pass Aleph 0s or 3 of similar power rating, I hear a definitely bigger impact in the bass and more presence (noise?) in the midrange that make the HE-6 sound more "there" to my ears. The specs on these early amps (including noise floor) are not nearly as good as those of their younger bretherens, but their minor flaws(?) seem to make them better suited for the HE-6. Go figure.
Finally, when I switched to older Pass's amps, such as the Stasis S150 (75 wpc into 8 ohms) and S300 (150 wpc) which are also Class-A but with a sliding bias--some still continue to proclaim that Nelson cheated here--including the new Taiwanese Usher R 1.5 monoblocks using a Stasis design, the bass became positively thunderous. Sure the higher power helps but there is a difference in the quality of the bass that is common to these amps and not shared by their admittedly smaller and purer bred siblings.
All this is my round about way of saying that why the incredibly sound purety of the latest Class-A amps is something worth seeking, it is not a panacea for the HE-6. You are left wanting just a little more bass and more presence in the midrange. I compromised and settle for the slightly less purified sound of the older Stasis design to get more forward presentation and a little more impact in the bass. There are a bunch of Statis amps circulating on eBay, from the S150 (perfect size for the HE-6) to the S300 and S500 ( if you want to melt the HE-6) and the more affordable Nakamichi PA-5 and PA-7, which can always be tweaked to sound more like a Threshold (bigger caps ; higher bias).
But I would stay away from mass-produced receivers/integrated amps by large manufacturers designed to produced great THD and large power rating and always endowed with beautiful front panel to impressed consumers at the expense of the sound. With the exception of the HK775 and HK 870, my foray into the mass-market amps of the 70s and 80s met with rather disappointing results.
