Besides one tube amp on long term loan in the 90's and my BHC the past 2 years (useless on my planars) I've been all SS since 1972. I'm a Nelson Pass SS guy.The HD600s and R70x are both 102dB/V. the HD600s are 2dB less sensitive (97dB/mW vs 99dB/mW although diyaudio put the HD600 at 100dB/mW) but the R70x has more impedance (470-900Ohms vs 300-600Ohms).
That could be the reason. Many say the HD600s sound better with tubes than solid state. That could simply be preference as I heard tubes are warmer than solid states.
The OTL tube impedance is high, which does give a boost to open ear headphones bass in the 60-100 Hz range. There is also the addition of harmonic distortion which does give in reasonable amounts a warmth that recalls acoustically warm classical settings vs typical sterile 2-D digital. That plus of course the soundstage from my OTL which actually has one unlike the 600 on SS which is blob. No SS amp is going to deliver those things, even if technically speaking the OTL "improvements" are arrived at via "impure" techniques/results.
Including room stereos I've probably heard more consumer electronics than anyone on this site - top 5 anyhow, and W.R.T. headphones a used HD-600 with new stock pads, a BHC w/ speedball (also possibly used) and a really good DAC (my case a Gumby) gives for everyone but bassheads a very fine experience for cheap money. Better than my $1695 Ragnarok.