Take a look here:
http://www.audioheritage.org/vbulletin/showthread.php?4557-B460
The last paragraph of text describes the operation, the last page is a graph of the drive.
I'm not familiar with your pre/pro, but that's a description of what the BX63A does. I suppose it wouldn't hurt to slap some serious + EQ in at 26Hz and give it a listen. It mentions that the EQ is centered on the vent tuning freqency, so I think that's the main clue as to what to look for.
http://www.audioheritage.org/vbulletin/showthread.php?4557-B460
The last paragraph of text describes the operation, the last page is a graph of the drive.
I'm not familiar with your pre/pro, but that's a description of what the BX63A does. I suppose it wouldn't hurt to slap some serious + EQ in at 26Hz and give it a listen. It mentions that the EQ is centered on the vent tuning freqency, so I think that's the main clue as to what to look for.