I think it really depends on when you say "LFs" which are of most importance to you. And to find that out, you really need to measure your room.
Have you downloaded and tried the Porous Absorber calculator?
See Porous Absorber Calculator V1.58
Here's what I've done. I currently have two main LF problems in my room - which I discovered through measuring the room. In my case, these problems are:
- modal ringing at around 40Hz
- a big broad dip from 90 to 150Hz
Knowing what areas are free for bass trapping in my room (and knowing the dimensions of these areas) I then used the calculator to find the best GFR to use. The calculator is handy because it has spin buttons which lets you see the changes more easily. To put this into an example....... the back half of one of my side walls forms a large alcove with a chimney breast. The alcove is 40cm deep. Using the PAC, I put 400mm into the "absorber thickness" field. I changed the "Start Graph at" field to 30Hz to see what's happening at the lowest frequencies. I changed "Absorber flow resistivity" to 2000. I then used the spin button to increase the "Absorber flow resistivity" and watched the graph changing. For me it was about compromise. As absorption at 40Hz increased, absorption in the 90-150Hz region decreased. So I found the best range of GSR which dealt with my frequencies of interest by doing this.
And as you can see, there was no "ideal" for LF absorption. One end of the spectrum was best for one area of LFs, but at the cost of the other.
But I should add a couple of other things to consider:
- depending on where you live, you may or may not be able to find MW/GF which has a published GSR. And even if you do, there's no guarantee that the published spec is correct. (That's where I'm at. I have my chosen GSR figure, but no product to buy with that figure).
- Even if you did know the GSR and it was accurate, things like the "facewise/edgewise" question, how the trap is constructed (and therefore how much the fibre is compressed), etc. will have an effect on the figure.
- many people say that the behaviour of LFs in room corners is different and the PAC does not necessarily allow for this. If so, that obviously changes the significance and/or accuracy of the info obtained from the PAC. IN fact, I believe some folk on this forum compared the behaviour of SSCs using a low GSR mineral and a more dense product (RW3/RS60/OC703 I think) and found the results to be less divergent than one might have expected.
So, lots of things to consider it would seem. Enough to give you a headache!
And yet, a lot of people knowledgeable in all things acoustics will recommend in threads on here that if your traps are going to be large, and you want them to deal with the LFs in your room, generic fluffy loft insulation will be the best choice. That's the advice I'm going to stick by. For the reasons given above, I can't really see an alternative.
One thing I might do though if I can get enough spare RS60 together - fill my alcove with fluffy, measure, replace for RS60, measure, stick the results up on this forum.