I’ve had the following experience with two pair of Sound Lab ELS/backplates:
I believe sometime around 1997 I bought a used pair of A-1s (90 degree dispersion) that were manufactured in the early-mid 1990s. The backplates had already been upgraded to Toroid 1s. No crates.
I used them with Audio Research tube amplifiers until last year when the panels developed issues. I communicated with the excellent people at Sound Labs and the decision was to upgrade the panels to PX Bass Focus (BF).
Coincidently at the same time a pair of used A-3PXs (45 degree dispersion) with Toroid 2 backplates and crates were being sold nearby so I bought them and used the A-3 crates to ship my A-1 panels (out of the frame) back to Gunnison UT.
I found the A-3PX Toroid 2 system to be excellent sounding, although lacking the extreme low bass when compared to the larger A-1s. My plan was use the Toroid 1 backplates (from the A-1s) with the A-3PX and the Toroid 2 backplates with the ‘new” A-1PX BF system.
Shortly before the A-1PX BF were to be shipped back to me I compared the two pair of backplates on the A-3PX speakers. The following statement applies only to my old (over twenty-five years) Toroid 1 backplates because other Toroid 1s might sound different than mine.
Within seconds I realized (my ears told me) I could not live with the old backplates. I tried to listen again, the next day. Same result.
I called Sound Labs and worked out getting another set of the latest backplates. Thats my quick summary of my experiences.