Those differences are exactly what I'm interested in discussing hifiman - and, to be accurate, if you re-read what I quoted and responded to above, DG is talking about how her 600 ohm beyerdynamic T1's sound with the gec tube.
In comparison to hd800's, many beyers have pretty flat impedance curves. If the T1 is in the same camp (headroom hasn't posted an impedance curve for them for whatever reason), they may well not get the bass boost from a higher impedance cathode follower as opposed to a 5998. For purposes of illustration, compare the hd800 and dt880 impedance curves:
The fact that the 5998 is bassier in DG's setup, and the GEC is bassier in mine, is probably of high relevance to others who may be trying to figure out which, if either of these tubes, is worth buying given their tastes and gear.