Benchmark has a nice excel file at the end of this article that you can use to calculate the effects yourself:
https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor
Personal comments: The thing is this, if you include a series resistor in front of the speaker (i.e. a long wire), you will weaken the electrical component of the speaker chassis (Qe), so you will get less control of the cone, and thus more output around its resonance frequency (Fs). (See for example:
http://speakerdesignworks.com/Electrical_Factors.html)
Now for surround channels that are high-passed anyway, it might not matter that much. However, I'd try to keep the total attenuation below ideally 0.2dB or maximum 0.5dB. That said, listening distance also plays a role, and if you can control the volume of the surround channels, you might even compensate for the attenuation.