The multi-pin cylindrical connectors provide 1 db of attenuation at 1 MHz and 42 db at 30MHz. The room itself is spec'd for 60 to 70 db. It would be preferable to maintain that level of isolation if it's not too expensive. The DB 25's are even worse, providing about 27db at 30MHz. How was the frequency span for testing decided.
Low end is determined by room.
To get quantitative measurements, need to have antenna at least a wavelength away from the device under test, to get in the far field of the antenna. Given that the range is 11 meters long, the low frequency limit is about 30MHz. Most interference is below this frequency, and the area of the feeds is getting quite small at this frequency. Also the expense of going higher in frequency is a consideration. Prefer 4 feed-thrus.