Until we have some reliable third party measurements of both speakers, I'd prefer to stick with the manufacturer's own ratings, conservative or not. RBH says 50-20,000 Hz ±3 dB for the
61-SE/R, and Salk says 42-20,000 Hz ±3 dB
for the ST.
This isn't the first time I've heard it said that the ST lacks midbass punch. I had that initial impression as well -- but I was wrong. Being used to listening to bass reflex speakers, the bass from a transmission line cabinet sounded deceptively quiet at first. Bass only appears when the recording calls for it. After I found a few recordings with some prodigious bass, that I hadn't heard with my previous speakers, I soon changed my mind.
After getting used to the STs, if I hear some bass reflex speakers in a direct comparison, the bass from the reflex speaker sounds muddy and exaggerated. They sound like the bass port is sounding off all the time at a low level, regardless of the musical content. This tends to make the midrange sound muddy. Of course, like all verbal descriptions when trying to compare the sounds of speakers, these words tend to exaggerate the actual differences.
Perhaps you are used to listening to bass reflex speakers as well as there are relatively few commercially available transmission line speakers available.
What do the 61-SE/R and SE-61 cost? As a guess, the 61-SE/R looks like it costs more. I've never seen nor heard them. In the east coast (Wash, DC and Baltimore area) I've never found a dealer who carries them.