Wanting to retrieve values from external workbook by searching name of column.

Hlookup is for specific cell returns. While it could be modified to do what you want, LARGE is the only one you need in the case of the largest number.

If you haven't named the Range then it would be something like:

=Large(Signals.csv!$B$1:$B$69,1)

If you have named the Range then it would be something like:

=Large(Signals.csv!SNR_average_dB,1

You can check the name with Ctrl+F3.
To name a Range, select the range you wish to group(click and drag over the area you want), and name it accordingly (example: SNR_average_dB)
The name name of any range you have selected will appear in the upper left corner of the Worksheet, just up and to the left of Cell A1
If you need to edit it, you will need to go to the Name Manager as mentioned above. For your reference the Name Manager is in the FORMULAS Ribbon
Cheers. I hope this helps.