That was a very tough question.
The HE-6 tends to produce hardness and glare in the midrange with many amps (and cables). To me, the minimum requirement for an HE-6 amp would be to deal with these flaws AND to have sufficient power. Many amps failed and only a handful succeeded. But I would say that the “best” amp for the HE-6 depends on your musical taste and sonic preferences. There is no clear overall winner here but, like farmers from Iowa, they are outstanding in their own field.
The GS-X Mark2 produced a smooth and well-articulated sound with silky transients and plenty of inner details without any trace of the hardness and glare typically associated with the HE-6. The feathery light and silky smooth transients of the GS-X Mk2 must be heard to be believed. The focus and imaging is very natural (neither forward nor recessed) and the soundstage was the largest I ever heard from the HE-6 endowing the sound with the presence, air and layers of live music. The bass was tight and solid with tremendous impact. This was one of the top four amps I heard with the HE-6. The greatest strength of the GS-X Mk2 is its ability to perform well with all kinds of headphones and all kinds of music.
With the Bakoon HA-21, the HE-6 sounded remarkably free as well of the hardness and glare often heard with other amps. There was plenty of bass with the HE-6 for Jazz and classical music, but I wished for a tad more impact here with rock music—this was a real surprise for a solid-state design; perhaps a bigger power supply is needed here but that may be quibbling. Where the Bakoon truly excelled was in the warm but also vividly detailed midrange. This is an uncanny sound quality that I have not heard from any other amps. If you like to hear warm vocals with vivid and rich details and smooth transients (guitars, violins…) within the music, this amp is ideal for you. The excellent way the Bakoon rendered female voices in particular is entrancing.
With the Woo WA5, the HE-6 delivered a smooth midrange with outstanding presence thanks to a forward imaging and very tight focus rivaling that of the LCD3/SR009. This sound was remarkably free of the hardness or glare usually associated with the HE-6. The slightly improved soundstage was larger than with the LCD3 but still constricted compared to the HD-800. I save the best for last: the bass, at once tight and weighty, a rare combination, was delivered with an authority that will rock your socks off. This was truly a match made in heaven for rock. Jazz- and Chamber-Music lovers will have much to like about the HE-6/Woo WA5 pairing as well.
With the EAR HP-4, the HE-6 also produced a musical sound free of hardness and glare. The focus of the HE-6, typically a little fuzzy, became pin-point sharp here; the center image was also pushed a little forward to give a greater presence to vocals, similar to the beautiful results with the Woo WA5. In addition, however, there was also increased air around the instruments and enlarged soundstage to improve the 3-D illusion. Surprisingly for tube design, while the bass was well controlled (read tight), very dynamic and perfect for classical music, it was not ideal for rock where a bid less control and perhaps more weight (mid-bass bloom) is desirable. Overall, the sound was detailed and dynamic, but also so open and relaxed that I frequently forgot to focus on it and was simply enjoyed the music. That’s the ultimate compliment I can give for an audio gear.
Sorry about the long discourse but I am reluctant to endorse or dismiss such great amps without sufficient explanation. I hope this helps you MAKE YOUR OWN DECISION.