I'd like to chime in about the HE-6/SR-71B discussion, so apologies if this is a bit off topic. And I'm coming from owning a HE-5 and an iBasso PB1, so please take it for what it's worth to you. My history of the HE-5 has been nothing short of love hate, hate because I had the most difficult time amping it properly (bass would clip on my once owned Purity Audio KICAS Caliente), and when it was amped properly (when I owned the EF-5), it still had a tad too much treble energy for my tastes. Well, I've been trying to part with my HE-5, but no bites. I knew I wanted a balanced amplifier someday when people described how it made the soundstage sound more three dimensional, and lo and behold here comes both Ray's Protector and iBasso's Toucan. I settled for the Toucan, and just to entertain myself I reterminated my HE-5 with a Hirose balanced plug. I'm not one for snake oil claims and your stereotypical "it changed everything" babble, but I was very very very very impressed when I heard the HE-5 balanced on the PB1. It was able to adequately drive the HE-5s, and not only were they dynamic, but for the first time the soundstage and imaging sounded coherent (which was an issue for me when the HE-5 was single-ended) The word that immediately came to my mind was "correct." The soundstage and imaging was for the first time and finally correct. Because of the inherent design of balanced, the background was noticeably blacker to the point where, for the first time since I could remember, I was hearing things in recordings that I've never heard before. All of this coming from this little portable piece of thing, which the volume pot not even close to max (hovers around 2-3 o'clock).
Now obviously this may have no relevance at all to the current discussion of the SR-71B...or the HE-6 for that matter. But if the SR-71B performs anywhere near the level of the PB1, and FWIR it ought to surpass it, then I would say that you're definitely in for a big treat when you pair the two together.