I got a pair of "used" Hifiman HE5LE's today and spent 3-4 hours listening to them on a couple of different amps on a wide variety of music...from electronica to bluegrass. I decided to write down just a few impressions from that time.
First, the planar phones are more transparent and detailed than the HD650's. This is not to say that there is a lack of detail on the 650's, but rather that some of the details...especially those related to upper midrange and high frequencies stand out in the mix "less".
The planar phones...to my ears sound the best on my tube amp. It fleshes the sound out a bit and warms ups the lower midrange. On my Headroom Home...which is a very neutral sounding SS amp, I find the LE's very revealing and at times almost bright...nothing like the
Beyer DT990, but things like rim shots, guitar pick sounds and high hats really stand out.
The 650 is a bit warmer sounding set of cans...regardless of SS or tube power. There is a slight midbass bump that contributes to the sound of the wooden body in a string bass that is not there in the HE. By the same token, the midrange on the HE is not "thin" as much as very neutral and "fast".
One thing I felt was different from many reviews is that with SS amplification (at least what I had) the bass was very taut on the HE, but a bit lacking in weight unless the recording had a strong bass line. There is a round, "purring" sound that a bass guitar being run through a big cabinet in a live setup has that was not as present on the HE's as on the 650's. By the same token, the HE was faster across the frequency spectrum. I'm listening to it right now with my ASL tube amp with premium matched EL-84's and a NOS, windmill getter 5751 driver and it sounds great on on some Acoustic Alchemy.
Tonally, the difference in the midrange with the two cans is not huge, but the perspective is more up close and intimate on the HE with a SS amp. OTOH, you move back a couple rows with the tubes but the sense of space and soundstage is better. Interestingly, on some well recorded 24/96 Mozart quartets, the transparency and up close perspective on the SS amp gave more of a "you are there" feeling.
Don't mean to waffle...the truth is I like them both although they are different. The 650 has romance and is easier to listen to poor recordings on. The HE has more clarity/transparency/speed and a better top end. It makes differences in amps stand out more which can be a good thing or a bad thing depending on your wallet

Looking forward the ChiUniFi so I can hear some different amps on these phones.
Hibuck....