Kinda surprised you gave it such a good rating given that it's a 6.5" woofer with very little bass, along with a quite a funky frequency response. When you say "the benefits of a sealed enclosure was there with its ability to play sub-bass without getting distorted", I'm quite surprised by that because sub-bass is below 60Hz and judging from the spinorama with the bass starting to roll off quite steeply at 90Hz I'm surprised there'd be any sub-bass to be heard at all? I would think this speaker would need to have a Low Shelf Bass boost via EQ of at least 5dB to even start noticing the sub-bass, but then you'd be pushing the speaker towards it's 94dB measured level which would put it into unhappy distortion territory? Given that this is a 6.5" woofer it should be able to play lower, and I don't think it really deserves it's awarded status.
EDIT: even on their spec page for the speaker they list the Frequency Response as 55Hz-20kHz, so that's not really covering the sub-bass below 60Hz
I think for sealed boxes you would get much less bass impact than same driver size ported speakers so it makes sense， for sub bass comment I have no clue also
Actually, you might be onto something re Room Gain (@Frank Dernie), I had been looking through reviews of 8" models to disprove you in saying well how come these well received speakers didn't sound boomy with their deep bass reach combined with the Room Gain that you're supposing. Well actually what I found in the following three 8" speaker review examples (see pics & links at end of post), during the listening tests Amir either put in a High Pass Filter to cut off the bass in the 308p review citing distortion issues, but potentially could have been confusion with Room Gain boominess. The 708p Amir chose to listen in farfield rather than on his desk, so perhaps speakers not so close to the wall in that one......so no adjustment to bass required. Adam T8V review he had on his desk & noted he had some boominess from the room due to low-playing speaker activating room modes. So it looks like you're onto something re Amir's listening situation and the room gain.....and it could be possible he confused boominess of the 308p on his desk with distortion but that latter point is just a coincidental theory. Going back to the review of the speaker in this review (the NHT C3), I think your Room Gain point does carry weight to explain adequate sub-bass.....however this speaker would still be underpowered in the bass if you didn't have Room Gain, so I don't particularly like it.Roll off is slower in closed boxes, so whilst the -3dB point is higher for a given box size the bass extends well in room because the wall and floor gain have "something to work with", ie some output to augment, whereas with a reflex the level below port tuning is so low it isn't augmented and around port tuning it can be excessive particularly if the room has a principle mode near that frequency.
A slow roll off in the bass, starting even at 100-200 Hz often gives good in room results and needs less/no room correction.